Background: The purpose of the study was to present components of fitness and body composition and to determine the correlation between parameters of biological development in a group of schoolchildren.Material/Methods: Analysis of somatic features of 12-year-old children, of both sexes, from the Pomeranian province in Poland (99 boys and 96 girls) included basic parameters of the respiratory system, body fat, height and body weight. Functional indicators, i.e. components of motor abilities, have been tested.Results: Boys had significantly better results of endurance and jumping ability, and girls dominated in flexibility and somatic components like height, weight and skinfold thickness and additionally in Forced Vital Capacity and Forced Expiratory Volume in One Second. In the group of boys the Maximal Mid Expiratory Flow25/75 was significantly correlated with shoulders strength. In the group of girls Forced Expiratory Volume in One Second and its relationship to Forced Vital Capacity was correlated with shoulders strength. The Peak Expiratory Flow rate was significantly correlated with abdominal strength and flexibility. Forced Vital Capacity was correlated with flexibility and Forced Expiratory Volume in One Second - with flexibility too. The biggest intensity of correlation was observed in the case of boys skinfold thickness with speed, jumping ability, shoulder strength, agility and endurance. The group of girls has weight correlated with speed and jumping ability and skinfold thickness correlated with the same parameters. Boys’ endurance of was significantly correlated with weight and skinfold thickness and girls’ endurance additionally with height.Conclusions: Significant differences between males and females have been identified with regard to body type and fitness. In the group of girls more correlations between parameters of the respiratory system and basic components of fitness were observed. However, the opposite situation appeared in the case of relationships between the remaining somatic indicators (height, weight and skinfold thickness) and general components of fitness, where more correlations in the group of boys have been reported.