Three genetic features were studied in the mussel Mytilus edulis L.: a) genetical differences between slow and fast growing larvae, b) genotype-dependent spawning time and c) inheritance of the intron-3 of the calmodulin gene-1 in juvenile mussels. a) Studies with juveniles have shown a positive correlation between growth and heterozygosity, and, in some cases, between larval growth and juvenile growth. The possible correlation between heterozygosity and larval growth was tested using allozyme electrophoresis. Larvae from different mating systems (half-sib families with a single female parent, half-sib families with a single male parent, reciprocal cross and mass mating) were selected into fast and slow growers when about l 0-30% were undergoing metamorphosis. Both groups were genotyped at the juvenile stage. Allozyme data from up to 12 loci showed generally good agreement with Mendelian inheritance. No significant differences either in heterozygosity or in juvenile shell length between the slow and fast growing larvae were found. At no loci were homozygotes significantly larger or smaller than heterozygotes w ithin each group of slow or fast mussels. In addition, no correlation between larval growth and juvenile growth was observed. These observations might imply that the loci scored are either uncorrelated with larval growth or that larval growth is independent of genotype at these loci. b) Heterozygote deficiencies are common m wild populations of marine bivalves, genotype-dependent spawning time has been proposed as a possible explanation for this deficiency. Mussels were collected every fortnight in two localities in the Menai Strait, North Wales during the spawning seasons of 1993 and 1994 and induced to spawn artificially. No significant differences in shell length between spawners and non-spawners were found. Allozyme analysis showed no significant differences in heterozygosity between spawners and non-spawners during the 1994 season. It was concluded that genotype-dependent spawning could not explain the heterozygote deficiencies observed in this population. c) Juveniles from the half-sib families with a single male parent were used to amplify the calmodulin gene-I intron-3 with PCR. Inheritance of this gene was confirmed to be in Mendelian fashion. Three alleles ( 460, 525 and 600) were found in mussels from the Menai Strait, and genotype frequencies agreed with the Hardy-Weinberg model when the less common alleles were pooled together. Further research in this area is recommended.