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Genetic analysis of 20 sterile mutants prone to form hyphae revealed 11 novel ste genes (sep6 to sep16) of Schizosaccharomyces pombe. None of the mutants was completely mycelial. Most mutants formed branching hyphae and showed normal septation. Aberrant septal structures and actin distribution were seen only at 36°C. sep9-307, sep14-576 and sep15-598 showed genetic interactions with sep1-1, a mutation in a forkhead transcription factor homologue. Additional genetic interactions were detected between sep6-194, sep15-598 and cdc16-116, a mutant allele of an anaphase modulator of p34cdc2. sep9-307 and sep15-598 caused dikaryosis in wee1− background. In mating and sporulation tests, sep6−, sep7−, sep9−, sep10−, sep11− and sep15− proved to be defective in conjugation only, whereas sep8−, sep13− and sep16− were also defective in meiosis-sporulation. sep12− and sep14− were only partially sterile. All mutants could produce M-factor but sep8−, sep11−, sep15− and sep16− were defective in P-factor production. The mutations in sep8, sep11 and sep16 suppressed the pat1-114-driven meiosis. All mutants were sensitive to the presence of higher concentrations of chloride in the medium and to short heat shocks. The diversity of the mutant phenotypes and the pleiotropic effects of the mutations suggest that these sep genes might act in, or interact with, a multiple overlapping network of regulatory modules. Copyright © 1999 John Wiley & Sons, Ltd.