In crosses between diploid strains of Chlamydomonas reinhardtii, the mode of chloroplast gene transmission was studied and correlated with the behavior of DAPI-stained chloroplast nucleoids in young tetraploid zygotes. Phenotypically plus (female) diploids with mating-type genotypes of mt (+)/mt (+) or mt (+)/0 (hemizygote) and phenotypically minus (male) diploids with genotypes of mt (+)/mt (-) or mt (-)/mt (-) or 0/mt (-) were obtained by either crosses or polyethylene glycol induced somatic fusions between haploid strains. When crosses were made between mt (+)/mt (+) or mt (+)/0 and mt (-)/mt (-) or 0/mt (-), the transmission of chloroplast genes occurred mostly from the female parent, indicating a typical maternal inheritance. In these cases the chloroplast nucleoids of one of the two parents disappeared within 2.5 h after mating. In contrast, when female diploids were crossed to mt (+)/mt (-) male diploids, the chloroplast genes from both parents were transmitted to the progeny at equal frequency. The chloroplast nucleoids derived from the two parents were conserved for at least 2.5 h after mating. It is concluded that the chloroplast genes derived from the heterozygous male (mt (+)/mt (-)) are protected from the degradation that takes place soon after zygote formation, by virtue of the presence of mt (+) gene although this allele is recessive with respect to the phenotypic expression of sexuality in diploids.