Purpose. It has been suggested that the critical swimming speed (CSS ) of young swimmers may be estimated by using two timed maximum exertion efforts at distances of 50 and 400 m. The aim of this study was to find out if the estimated CSS for a group of boy swimmers corresponds to the results obtained from a 12-min swim test and to examine if there was a difference whether these tests were completed using different swimming strokes. Methods. The study was carried out on 24 boys (age 12.2 ± 0.1 y, height 158.0 ± 1.8 cm, weight 47.7 ± 2.2 kg), all of whom were competing at the regional level. The participants were timed completing the 50 and 400 m distances at maximal effort, while the 12-min test was assessed by the total distance swum, all three trials performed in the front crawl and breaststroke. Results. The results found a close relationship between CSS determined by the 50 and 400 m distances and the distance covered during the 12-min test for both strokes (breaststroke r = 0.79, p = 0.0000; front crawl r = 0.83, p = 0.0000). There were no significant differences between CSS and the mean velocity of the 12-min swim test with swum in the front crawl (0.862 ± 0.027 m · s-1 and 0.851 ± 0.027 m · s-1, respectively); however, CSS was significantly higher (p = 0.002) than the mean velocity found in the 12-min test in the breaststroke (0.769 ± 0.018 m · s-1 and 0.727 ± 0.022 m · s-1, respectively). Conclusions. CSS estimated on the basis of the front crawl but not breaststroke is a good predictor of the average velocity of the 12-min swim test for young male swimmers.