Based on countings and mark-release-recapture experiments the effects of intestinal eugregarines and ectoparasitic water-mite larvae on zygopteran imagines were studied in habitats on the W coast of Norway, In Enallagma cyathigerum and Coenagrion hastulatum, the larval mites attach during emergence: imagines on their maiden flight revealed the highest numbers of attached mites; the figures appeared to decline in the course of adult life. Heavily loaded individuals disappeared during the prereproductive and the succeeding reproductive phase. Pyrrhosoma nymphula and Lestes sponsa on their maiden flight carried low numbers of mites; the infestation increased consequent on repetitive visits to the water for reproduction, and the number of mites tended to rise with increasing adult age. In all Zygoptera the load of gregarines increased with advancing adult age owing to recurrent ingestion of infective gregarine oocysts; the gregarines influence zygopteran longevity. No individual carrying very high numbers of both gregarines and mites was ever recorded. Parasitism by mites and gregarines influences community structure of zygopterans through selective pressure against heavily-loaded hosts. — In wet cool summers there was virtually a non-appearance of propagating stages of the gregarines. Production of infective gregarine oocysts depends on the heat budget of the zygopteran flying season: the intrahost processes leading to union in syzygy demand ambient air temperatures of 18-20° C or higher, while the extra-host development of oocysts over the subsequent 2-3 weeks requires temperatures not much below. As for damages inflicted by mites, a corresponding dependence on warm weather could not be demonstrated. Ruptures of the midgut wall by gregarine overinfection involve injuries and partial dissolution also of the zygopteran epidermis, which is recognizable in the field as a discoloration of the zygopteran abdomen. Lesions to the midgut wall by gregarines and epidermal damage by mites were most tangible during periods of fine weather; enfeeblement of the hosts is presumed to be due to desiccation.