This study examined the effects of resistance training on muscular strength and jump performance in young male swimmers. It was hypothesized that adaptations would be of a lower magnitude in less mature (Pre-peak height velocity [PHV]) than in more mature (Post-PHV) subjects. Fourteen Pre- (-1.8 +/- 1.0 years) and 8 Post-PHV (1.6 +/- 0.5 years) swimmers undertook a 30 minute, twice-weekly resistance training program for 8 weeks. They were compared with matched control groups (Pre-PHV: -2.0 +/- 1.1, n=15; Post-PHV: 1.2 +/- 1.0, n=7). The effects on lower body isometric strength (LBS), measured with mid-thigh pull, and vertical jump (VJ) height in the Post-PHV group were large (effect size: 1.3 [0.4 to 2.2]) and small (0.4 [-0.4 to 1.2]) respectively. Effects on LBS and VJ height in the Pre-PHV group were moderate (0.8 [0.1 to 1.4]) and trivial (0.2 [-0.5 to 0.8]) respectively. Estimates in the Post-PHV control group (LBS: 0.7 [-0.2 to 1.6]; VJ: 0.2 [-0.7 to 1.0]) and the Pre-PHV control group (LBS: 0.1 [-0.5 to 0.7]; VJ: -0.3 [-0.9 to 0.3]) may indicate the extent to which maturation could contribute to the performance changes seen in the respective training groups. LBS and VJ are trainable, but to different magnitudes, in Pre- and Post-PHV swimmers. Following appropriate foundational training to establish technical competency, twice-weekly resistance training sessions of 30 minutes duration, comprising 3 sets of 4 exercises can be effective in Pre-PHV and Post-PHV youth.