 Benzoylphenyl ureas, including novaluron, do not inhibit chitin synthesis in cell free systems or block the chitin biosynthetic pathway in intact larvae.The precise biochemical activity of these compounds, that gives them their insecticidal activity, has not yet been elucidated.The most likely hypothesis is that benzoylphenyl ureas interrupt the in vivo synthesis and transport of specific proteins required for assemblage of polymeric chitin.