BACKGROUND: Young swimmers are rarely involved in training control, leading to inadequate volume and prescription of training intensities. This study aimed to examine the effect of prescribing and monitoring training intensities around critical velocity (CV) on 400 m frontcrawl performance during an 8-weeks period. METHODS: Twenty young male swimmers (13.4±1.0 years) were randomly assigned into two groups. The experimental group (EG) used training monitoring based on intensities around CV (95, 100, 105 and 110% of CV), while for the control group (CG) different heart-rate (HR) zones (EN1+, EN2, EN2+ and EN3) were used as an intensity criterion. The training volume was the same for both EG and CG. A maximal 400 m front-crawl test was performed before and after the 8-week period to determine CV intensities and to assess alternations in performance. The rate of perceived exertion (RPE) and HR values were also evaluated. RESULTS: Significant main effects of time were observed for T400, CV, HR and RPE. A significant group x time interaction was found for T400 (p=0.035, effect size=1.07, moderate). Results revealed significant pre-to-post improvements for both EG (from 328.46±26.25 to 305.88±20.09) and CG (from 329.07±25.24 to 314.75±19.87). A significant group x time interaction was also found for CV (p=0.005, effect size=1.51, large). Results revealed significant pre-to-post improvements for both EG (from 1.126±0.08 to 1.207±0.07) and CG (from 1.124±0.08 to 1.173±0.07). CONCLUSIONS: Both training methods induced significant improvements in 400 m front-crawl performance with a greater degree for the training prescription and monitoring using CV intensities.