A reciprocal mussel seed transfer experiment was conducted involving two farm sites (Charles Arm and Thwart Island) whose seedstocks differed significantly in their respective compositions of Mytilus edulis L., M. trossulus Gould and hybrids. Electrophoretic variation at the mannose phosphate isomerase (Mpi) enzyme locus was used to classify the two species and their hybrids. Survival and growth in shell length, wet weight, shell weight and flesh weight were monitored over a 14-mo period. Both stock groups originating from Thwart Island seed (TI reared on its native site and the transferred TI-X stock) and the transferred stock group originating from Charles Arm seed (CA-X) showed no evidence of genotype-dependent variability in survival whereas, in the Charles Arm seed reared on its native site (CA), the proportion of M. edulis declined in relation to M. trossulus. There was no evidence of either a survival advantage or disadvantage in hybrids. After 14 mo, both transferred stocks (CA-X and TI-X) had comparatively higher frequencies of M. edulis and hybrids and lower frequencies of M. trossulus than did their respective nontransferred stocks (CA and TI) indicating a significant genotype x site interaction effect on survival. Significant species, stock, stock × time, species × time and stock × species × time effects on shell length, wet weight, shell weight and flesh weight occurred. Intrastock growth rates of M. trossulus were lower than M. edulis and hybrids for shell length (3 of 4 stocks), and for the 3 weight variables, wet weight, shell weight and flesh weight (4 of 4 stocks). These results support the conclusion that growth is genotype-dependent in rope-cultured mussel populations and that these differences in weight growth favoring M. edulis are maintained when such stocks are transferred to other sites. Mussel biomass (kg m⁻¹) was similar for the CA, CA-X and TI-X stock groups and lower for the TI stock group. Rates of production (wet weight) over the 14-mo growout period were similar for the CA, CA-X and TI stock groups and significantly higher for the TI-X stock group. In both stock groups originating from Thwart Island seed (TI reared on its native site and the transferred TI-X stock), the intrastock rates of production of M. edulis and hybrids were significantly greater than sympatric M. trossulus. However, in the transferred Charles Arm stock (CA-X), the rates of production were similar in M. edulis and M. trossulus and significantly greater in hybrids. In the Charles Arm stock reared on its native site (CA), the rate of production of M. trossulus was significantly greater than either M. edulis or hybrids. We conclude M. edulis and hybrids have intrinsically greater rates of weight growth, but not necessarily length growth, than does M. trossulus. Neither M. edulis nor M. trossulus nor hybrids have consistently greater rates of survival or production in suspended rope culture. These results are discussed in the context of a directed seed-stocking program involving the transfer of M. edulis seedstocks to sites whose native stocks have a high M. trossulus component as a means to enhance commercial mussel production within the