The genetic basis of slow growth rate and aberrations in the ciliary pattern was studied in the multi-left-marginal variant of Paraurostyla weissei. The 3:1 segregation in F2 sibling crosses and 1:1 segregation in test crosses indicate that the aberrant phenotype is controlled by a recessive allele at a single gene locus termed mlm. The phenotypic change from wild type to mlm takes place about 5–8 cell cycles after conjugation. The study established that total conjugation in P. weissei is a true sexual process in which meiosis, fertilization and Mendelian segregation occur.