In ''City of Nets,'' his insightful 1986 book about Hollywood in the 1940's, Otto Friedrich describes how Rita Hayworth became a movie star after she changed her name from Margarita Carmen Dolores Cansino and took care of her ''hair problems.'' Her thick dark hair and low hairline were deemed too Hispanic, but nothing that couldn't be fixed with red dye and electrolysis.

Los Angeles had deemed itself too Hispanic as well. Though the city had once belonged to Mexico, it wasn't proud of its heritage. Municipal swimming pools carried ''Anglos only'' signs. Some stores conducted their racism with an oddly apologetic note: ''Sorry, no Mexicans.''

While Hayworth conformed with her hair, young Mexican-Americans rebelled with their clothes. Imitating African-American style, they danced to jazz and wore ''zoot suits,'' baggy pants, long jackets, big hats -- and this was long before mass-marketing would subvert radical fashion. No Phat Farm for zoot suits. Their parents disapproved; white Los Angelinos were scared of these ''pachucos'' or punks.

Racial tension met wartime apprehension in 1943, when the city was overrun with 50,000 sailors about to embark for combat overseas. The sight of young Mexican-American men with attitude, dressed in outlandish clothing, brought out the worst impulses in equally young military men already pumped to feel hostility toward aliens. With sailors stationed near the barrio and regularly traipsing through Mexican neighborhoods, the situation was an explosion waiting to happen.

The detonating event appears to have been a fight instigated by a perceived insult. For a week Los Angeles produced a real-life spectacle in which United States armed forces beat up Mexican-American civilians. Needless to say, those images didn't appear on war bond posters. ''Zoot Suit Riots'' recalls this nasty chapter on PBS's ''American Experience'' on Sunday at 9 p.m.

The zoot suit riots were part of a wave of racial violence in the 1940's when fighting broke out between blacks and whites in Detroit, New York and St. Louis. But the significant minority in Los Angeles was Mexican, not African-American, and these abuses haven't been nearly as well documented. (In ''City of Nets,'' Mr. Friedrich recalls ''The Ox-Bow Incident,'' a 1943 film based on Walter van Tilburg Clark's novel about a lynching. The film fails to mention that the victim was Mexican.)

Joseph Tovares, producer and writer of the PBS documentary, has stylistic ambition that sometimes obscures the significant story he's telling. He couldn't resist the urge to put film noir touches on his history lesson, set in the 1940's when the genre was most popular. Re-enactments are dimly lighted; jazz underscores the mood. The program zeroes in on a 1942 murder case, in which one young Mexican-American man was accused of killing another, and tries to link it to the zoot suit riots.

The flourishes feel forced, as Mr. Tovares strains to keep the murder-mystery idea afloat while having experts and witnesses explain the wider sociological and historical currents that fed the riots. The dramatic line goes slack and the details get muddied, but not enough to obscure the value of the enterprise. It's a jolt to see film footage showing servicemen stripping young men -- boys, really -- on the street, and burning their zoot suits. Soon after that, sailors felt free to beat any Mexican, zoot suit wearer or not.

You can see why Mr. Tovares was drawn to the murder story, with its pulpy tone and pulpy name -- the Sleepy Lagoon case, after the place where the killing occurred. The murder itself had a dreary familiarity -- a fight, a death -- that would ordinarily have gone unnoticed, dismissed as a gang-related event. But the tabloid press saw an opportunity to needle zoot suit wearers. When some youngsters were arrested, they were dubbed ''baby gangsters'' and the case became a cause célèbre, held up as an example of Mexican hoodlumism.

The details are murky and the defendants were not innocents. Many of the 23 men had been arrested before and weren't above settling an argument with a fight. But their trial became a classic miscarriage of justice. The judge wouldn't let the defendants clean up before trial, so they appeared in court unkempt and scary looking. They were convicted and then became a different kind of cause célèbre. Hollywood figures, including Orson Welles and Rita Hayworth, lent their names to a special defense committee. Eventually, justice prevailed. The verdict was overturned on appeal.

Certainly, the racial assumptions that led to the unfair verdict were part of the same poisoned atmosphere that gave rise to the riots. When they were over, a special commission concluded race was the problem, but the Los Angeles mayor insisted it was juvenile delinquency. The city council made its own fashion statement. Wearing zoot suits became a crime -- real, not aesthetic -- warranting a jail sentence. They vanished, like Rita Hayworth's Hispanic hairline.


Zoot Suit Riots

On most PBS stations Sunday night

(check local listings)

Written, produced and directed by Joseph Tovares; director of photography, Michael Chin; edited by James Rutenbeck; music by Claudio Ragazzi; associate producers, Desirée Garcia and Mylène Moreno. Hector Elizondo, narrator. ''American Experience'' is a production of WGBH, Boston; Margaret Drain, executive producer; Mark Samels, senior producer.

Photo: Los Angeles youths of the 40's, from the film ''Zoot Suit Riots.'' (Carlos Espinoza/WGBH)