Relative growth rates of six genetic groups of common carp were compared in small netting cages and in earthen ponds. These groups of carp included an isolate of the Chinese 'Big Belly Carp', its crossbred with a European isolate, and four European progenies, purebreds or crossbreds. Five different environments were produced in the ponds, mainly by varying the stocking rates of carp. Each of the (ten) cages was treated as a different environment. Each cage and each pond were stocked with random samples of each genotype, i.e., 'communal testing' was carried out. The performance of a given genotype in a given environment was estimated from its weight gain in that environment. The mean weight gain of all groups, stocked into a given environment, was taken as an estimate of that environment as it influenced the growth of carp. The characteristics of the regression of growth of a given genotype on the environment in which it grew [i.e., the coefficient of regression (b) and the Y intercept (a)] are taken as measures of its adaptation to that environment. No real differences in adaptation to pond versus cage conditions were isolated for four of the tested groups, the Chinese x European crossbred, the two European crossbreds and one of the European purebreds. The Chinese carp showed a specific adaptation to growth in ponds (or a lack of adaptation to growth in cages), whereas Dor-70 was specifically adapted to cage conditions. These results may be explained by the genetic history of the two lines. The Big Belly Carp was domesticated under conditions of Chinese subsistence aquaculture, which apparently generated an adaptation to gathering and utilizing natural foods. These are prominently absent in cages. Dor-70 was produced in a long-term selection experiment, which apparently generated a response for growth in cages. These results may be of applicative value, if common carp were to be considered as candidates for commercial cage aquaculture. It would then be important to use strains like Dor-70, which are adapted to these conditions, and avoid strains like the Chinese Big Belly carp.