Sexual crosses and somatic fusions were performed between complementing wall-less arg(-) mutant strains bearing chloroplast markers for resistance to antibiotics. The mode of chloroplast allele transmission was investigated in the diploid colonies developed from both vegetative zygotes and fusion products. Before mating or fusion, one or both of the parental strains were grown for 4 or 8 days on agar containing 5-fluorodeoxyuridine (FUdR, 0.1 to 1.0 mm), which selectively reduces the amount of chloroplast DNA in Chlamydomonas. When one parent was pregrown on FUdR, the frequency of vegetative zygotes transmitting chloroplast alleles of both parents (biparental or BP zygotes) decreased, the reduction being more drastic when the mt(-) parent was treated. Transmission was mainly uniparental maternal (UPm) or paternal (UPp) depending on whether the mt(-) or the mt(+) parent was pregrown for 8 days in the presence of 1.0 mm FUdR. Treatment of both parents led to a strong maternal transmission. In the experiments involving somatic fusion between parent 1 and parent 2 (same or opposite mt), the ratio UP(1)/UP(2), which was approximately equal to 1 in the control, decreased or increased according to whether the cells of parent 1 or 2 were pregrown on FUdR. In parallel, the frequency of BP fusion products always decreased. When both parental strains were treated with FUdR, the frequency of BP fusion products also decreased and the ratio UP(1)/UP(2) was roughly equal to 1. The effect of FUdR can be interpreted in terms of reduction of the input frequencies of parental chloroplast genomes at the time of gametic or somatic cell fusion, the bias in favor of the maternal parent being operational only in sexual crosses.