Two hundred and seventy-nine Polyzonium germanicum individuals of various stadia were collected seasonally in winter, late spring-early summer (the breeding period) and early autumn. Fresh weight, dry weight and defatted dry weight were determined for each specimen. Seasonal changes in water content and fat content, expressed as percentages of defatted dry weight, were analysed in relation to body size and sex. Changes in water content were more pronounced in the smallest individuals (juveniles), whereas changes in fat content were more pronounced in the largest (adults). Seasonal fluctuations in water and fat content were similar in both sexes despite the sexual dimorphism of body weight. The results suggest the following relationships within the species life-cycle: (i) water content increases during the warm months (up to 295% for juveniles in early autumn), which coincides with the moulting period, (ii) Fat content increases subsequently to moulting; overall maxima are reached in early autumn (population mean = 62–5%), as well as in early summer for the young, (iii) Both water and fat content decrease (to about 150% and 50%, respectively) in winter; however, there is no evidence of starvation–at least in ovigerous females which increase in dry weight during that season, (iv) Fat content strongly decreases (below 30% in the largest specimens) in adults of both sexes during the breeding period.