The extensity of digestive tract infection with different parasites in domestic fowl kept on large and small farms was diagnosed in individual months of the years 1986-1989. The obtained results were confronted with the values of average monthly temperatures and rainfall sums in individual months of the above-mentioned period. The numbers of birds examined in individual months of the years 1986-1989 were as follows: 105 to 234 from large farms and 32 to 112 from small farms. The numbers of examined chicks from small farms were lowest in winter, and on the other hand highest in summer. The numbers of examined fowl from large farms varied from month to month. Particularly Coccidia (average extensity 12.1%), scarcely Heterakis gallinae (0.2%) and Capillaria spp. (0.1%) were detected in birds kept on a large scale. The occurrence of Coccidia fluctuated from 5% in birds dissected in May, July and November to 22% in birds dissected in August. H. gallinae and Capillaria spp. occurred rarely, namely in the second half of the year after warm and rainy periods (Figs. 1 and 2). The occurrence of gastrointestinal parasites in domestic fowl kept on small farms is variable within the year while the change begins in summer and culminates in autumn after warm and rainy months. Only following the sporulation of larger numbers of oocysts, or the maturation of larger numbers of eggs or cysticercoids their pathogenicity can play a key role while their concentrations in the environment are increasing. Hence parasitoses begin to appear after the rise of temperatures and rainfall.(ABSTRACT TRUNCATED AT 250 WORDS)