To find out differences in speed, power, agility, endurance and BC in elite youth soccer players at the beginning and end of the season depending on the age category. METHODS: The elite players of 2 age groups (U21=24 and U18=21) of the highest league participated in fitness tests at the beginning (PRE) and end (POST) of the season. FP was assessed using the following tests: treadmill VO2max test (VO2max), YoYo test (YoYoIRT1), BC ratio of extra- and intracellular mass (ECM/BCM), power jump test: countermovement jump with free arms (CMJFA), countermovement jump (CMJ), squat jump (SJ), linear running speed at 5 m (S5), 10 m (S10) and flying 20 m sprint (F20), agility K-test, repeated agility ability (RAA). FP differences regarding age and time were examined using mixed model of MANOVA. RESULTS: MANOVA showed a significant effect of both main factors on FP (Age: F11,76=3.28; p<.01, η2=0.32, Time: F11,76=2.53; p<0.05, η2=0.25). The time had a significant effect on FP in following indicators: YoYoIRT1 (F1,43=43.43; p<0.01, η2=0.50), ECM/BCM (F1,43=8.23; p<0.01, η2=0.16), CMJFA (F1,43=4.35; p<0.05, η2=.09), CMJ (F1,43=6.66; p<.05, η2=0.13), SJ (F1,43=12.16; p<0.01, η2=0.22), S10 (F1,43=4.83; p<0.05, η2=0.10), F20 (F1,43=46.72; p<0.01, η2=0.52), K-test (F1,43=14.04; p<0.01, η2=0.25) and RAA (F1,43=7.04; p<0.05, η2=0.14). No significant changes were detected in VO2max and S5 (p>0.05). Between group effects of FP was significant in all tests except to S10 sprint. The highest improvement was in YoYoIRT1 (U21: PRE 2053.22+-322.96 m vs. POST 2233.33+-331.24 m, t23 = -3.70, p<0.01; U19: PRE 1683.81+-393.94 m vs. POST 2085.71+-389.93m, t20 = -5.27, p<0.01). CONCLUSION: The level of FP was the higher at the end of the competitive season. The level of endurance detected in both laboratory and field conditions revealed different values. Age appeared to have a significant effect when comparing the selected FP parameters