We exploited the inherent individual diversity in swimming performance of rainbow trout Oncorhynchus mykiss to investigate the hypothesis that maximum cardiac performance is linked to active metabolic rate (AMR) and critical swimming speed (U crit). Six hundred juveniles (body mass approximately 150 g) were screened using a swimming challenge of 1.2 m s(-1) to identify 'poor swimmers' and 'good swimmers', i.e. the first and last 60 fish to fatigue, respectively. These 120 fish were individually tagged and then reared in common tanks for 9 months, where they grew at similar rates and achieved a similar body mass of approximately 1100 g. Critical swimming speed (U crit) was then measured individually in tunnel respirometers, with simultaneous recordings of cardiac output via a ventral aortic flow probe. The group of individuals that were screened as poor swimmers remained so, with a significantly (27%) lower U crit than good swimmers [89+/-10 cm s(-1) vs 123+/-5 cm s(-1) (mean +/-s.e.m.), respectively, N = 6], a 19% lower AMR (147+/-12 micromol min(-1) kg(-1) vs 181+/-11 micromol min(-1) kg(-1), respectively), and a 30% lower maximum in vivo cardiac output (47.3+/-4.7 ml min(-1) kg(-1) vs 68.0+/-5.2 ml min(-1) kg(-1), respectively). When cardiac performance was compared with an in situ heart preparation, hearts from poor swimmers had a significantly (26%) lower maximum cardiac output (45.9+/-1.9 ml min(-1) kg(-1) vs 56.4+/-2.3 ml min(-1) kg(-1), respectively) and a 32% lower maximum cardiac power output at a high afterload (3.96+/-0.58 mW g(-1) vs 5.79+/-1.97 mW g(-1), respectively). Cardiac morphology was visualised in vivo by Doppler echography on anaesthetised individual fish and revealed that poor swimmers had a significantly more rounded ventricle (reduced ventricle length to height ratio) compared with good swimmers, which in turn was correlated with fish condition factor. These results provide clear evidence that maximum cardiac performance is linked to AMR and U crit and indicate that a simple screening test can distinguish between rainbow trout with lower active metabolic rate, U crit, maximal cardiac pumping capacity and a more rounded ventricular morphology. These distinguishing traits may have been retained for 9 months despite a common growing environment and growth.