The single-copy gene snu4, which encodes the small nuclear RNA (snRNA) U4, has been cloned and sequenced. Schizosaccharomyces pombe U4 is 128 nucleotides in length, similar in size to vertebrate U4 and shows substantial primary and secondary structure homology. The gene lacks sequences closely resembling vertebrate snRNA transcription signals, but has a TATA box at -33 to -30; TATA sequences flanked by several additional conserved nucleotides are found in the same position in the 5' regions of other snRNA genes from Schiz. pombe. The cloned snu4 gene was disrupted by transposon mutagenesis and used to replace one chromosomal copy of snu4 in a diploid strain. On sporulation snu4- haploid strains could not be recovered, demonstrating that U4 is required, at least for spore germination. Haploid snu4- strains are viable if they also carry snu4+ on a replicating plasmid but are unable to loose the plasmid under non-selective growth, demonstrating a continuous requirement for U4 for viability.