A 3-year study was conducted of growth and condition of northern largemouth bass Micropterus salmoides salmoides and F1hybrid largemouth bass (male northern smallmouth bass M. dolomieui dolomieui x female largemouth bass) stocked together in diverse pond treatments. Six drainable 0.4-hectare ponds in southern Illinois were initially stocked with similar numbers of half-sibling largemouth bass and F1hybrid larvae produced in the laboratory. Most populations were censused and restocked each spring and fall through three growing seasons (1979–1981). Supplementary data were generated by introducing age-1 largemouth bass and F1hybrids into two larger ponds containing mature populations dominated by largemouth bass and bluegills. Growth of pure largemouth bass was significantly faster than that of half-sibling F1hybrids in all populations studied each year, regardless of treatment, although third-year growths in the 0.4-hectare ponds were complicated by a differential response to tagging. After the initial stocks of pure and F1hybrid bass reached age 1, samples of their pond-spawned, age-0 progeny were subjected to starch gel electrophoresis to distinguish pure largemouth bass from those containing smallmouth bass alleles (F2and backcrossed individuals). In all cases, growth by pure age-0 largemouth bass was equal to or significantly greater than that by fish having smallmouth bass alleles (P < 0.05).