<bold>Purpose.</bold> During a biological year one can observe the effects of seasonal change on the number of functions and processes in the human organism. The rhythmical seasonal changes of a year are directly connected with a number of the functions of the human body. In humans, the first six months of a year are characterized by increased sympathetic nervous activity, while the next six months are characterized by increased parasympathetic nervous activity. As such, both somatic and motoric changes lead to periods of either high or low physical efficiency. The aim of this study was to assess what changes occur in the morphological and functional development of 14-15 year-old girls and boys across an entire year. <bold>Methods.</bold> The study of the seasonality of motoric fitness described herein was of a longitudinal character and was conducted five times, with a three-month interval between each study (1 - June, 2 - September, 3 - December, 4 - March, 5 - June) during 2004-2005. The study was conducted at the No. 1 and No. 7 Lower Secondary Schools in Opole, Poland, where 452 students (216 girls and 236 boys) participated in the study. <bold>Results.</bold> The results of this study found that there are two phases of increased levels of physical efficiency, which are particularly pronounced in March and June. From the results of this study as well as from results obtained from other available studies, it was found that the transition periods between seasons, especially during the transition between the first and second half of the year, are particularly unfavourable to the human organism. Having assessed these particular periods of the year (on the basis of a participant's training load), it seems necessary to emphasize that this transition is the most pronounced between the autumn and winter seasons. Significant seasonal changes were found in the general metabolism of the body, which is the most visible in regards to somatic development. An increase in the body's height and weight among teenagers starts abruptly in the spring season, reaches its peak in the summer, then it decreases in the autumn season and goes up again in winter. The study shows that from April till September a build-up of tissues occurs in the human organism, while from October till March the body stabilizes and uses its accumulated reserves. In the spring, the human body regenerates its central nervous system as it is influenced by sun and warmth and produces particular substances which function as links in the energetic and regenerative processes of nerve tissue. <bold>Conclusion.</bold> Seasonal changeability in the human organism leads to differences in levels of physical fitness. It is necessary to emphasize the character of seasonal changes on the physical fitness of boys and girls at the age of puberty. Through analysing intersexual variability of motoric skills it is possible to determine the exact direction of these differences. In tests that measured hand movement as well as body agility and flexibility, the female subjects achieved better results, particularly after the winter season. In regards to the scores achieved in the remaining Eurofit tests, the male subjects were at the forefront when compared to the females. On the basis of the research material presented in this article, it is claimed that the seasonal rhythm of the physical efficiency of the human body is also reflected in changes of motoric fitness. Thus, it can be considered appropriate to continue studies in this field in order to understand all and any phenomena and regularities.