The relationship between anthropometric and bio-energetic data and timed performance over 50 to 400 m was studied in 25 young male swimmers [11.3 (SD 1) years]. Anthropometric measurements included height, body mass, body fat mass, body area, thoracic section area (A _ts) thoracic circumferences, lengths of upper limb, bi-acromial and bi-iliac diameters. Maximal oxygen consumption ( [(V)\dot]\dot V O_2max; direct method), maximal anaerobic power ( [(W)\dot]\dot W _{an, max}; force-velocity test) and mean power in 30 s sprint ( [(W)\dot]\dot W _30s; Wingate test) were also measured. Each of these bio-energetic variables was expressed in absolute terms, relating to body mass, body area andA _ts. The stepwise regression method was used to determine contribution of the variables (anthropometric and/or bio-energetic) of the time achieved over the distance. The [`[\dot]] W\bar \dot W _30s/A_ts accounted for 46% of the time over 50 m (negative correlation). The [(V)\dot]\dot V O_2max/A _ts and height were negatively correlated with the times of performances over 100 m, 200 m and 400 m, these two variables accounted for 71% to 77% of the performance. These results would indicate that even in young boys, anthropometric and bio-energetic characteristics are both important in swimming performance, particularly the bio-energetic variables expressed perA _ts.