We have obtained five cDNA clones which are complementary to transcripts that accumulate specifically during the early stage of zygote formation in Chlamydomonas reinhardtii. Based on the time-course of transcript accumulation of their corresponding genes, these clones were classified into two groups: an early expression group (three clones; pZS102-1, pZS102-69, pZS1802) and a late expression group (two clones; pZS1860 and pZS1886). The levels of transcripts in the former group reached a maximum 30 min after the onset of mating, whereas those in the latter group peaked at 3 h. The addition of 10 micrograms/ml of cycloheximide reduced the decline in the levels of transcripts, except in the transcript corresponding to pZS102-69. Two clones, pZS102-1 and pZS102-69, contained highly-homologous predicted open-reading frames, and were judged to be cDNAs for two members of a novel gene family. These genes were referred to as zys1A and zys1B, respectively.