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Variation, transmission, and selection at 24 microsatellite loci are studied in five experimental families of the Pacific oyster (Crassostrea gigas). Two families are from naturalized North American stocks, and three come from Australian stock. As expected, there are multiple alleles at these loci and their segregating variation is reduced to four alleles or less in full sib progeny groups. Two to 21 loci were tested per family. Eight of the 24 loci have only codominant alleles, but 16 loci also have non-amplifying or null alleles. Of the 172 (43 × 4) parental sequences that were progeny tested, 30 (17%) were null alleles. Null alleles segregate in both Australian and North American stocks and their presence is heterogeneous among crosses. Overall null allele frequency in North American crosses was estimated to be 11% (eight of the 72 alleles progeny tested), just significantly less than the 22% (22 of the 100 alleles progeny tested) in the Australian stocks (P = 0.04). After accounting for nulls in genetic hypotheses, selection in the form of significant deviations from Mendelian expectations is observed in 16 of 43 progeny tests (37%). There is no systematic association between null alleles and selection, but analysis of dominance by sequential G-tests reveals non-additive kinds of zygotic selection. This has also been recorded in two other oyster species and the blue mussel. It appears that null alleles at microsatellites and selection near genetic markers are expected phenomena when studying transmission of genetic markers in bivalve molluscs. The implications of these results for breeding, aquaculture, and population genetics are discussed. [ABSTRACT FROM AUTHOR]