Plectrocnemia conspersa (Curtis) and Sialis fuliginosa Pict. are two well-known streamdwelling carnivores with contrasting predator strategies. The net-spinning caddis larva, Plectrocnemia conspersa, can be considered as a 'sit-and-wait' predator, while the alderfly larva, Sialis fuliginosa, is more active. Their common occurrence in a small iron-rich stream was investigated after preliminary observations indicated that the diversity and abundance of available prey in the benthos was low. Information on the role of terrestrial food for invertebrate predators appears to be lacking, although trout have been shown to take a significant proportion of terrestrial invertebrates in their diet (Elliott 1967). Attempts were made to determine and account for the abundance and distribution of Plectrocnemia conspersa and Sialis fuliginosa and their prey and to describe the factors affecting feeding by these predators. There is evidence that many herbivores and detritivores in streams respond to factors related to food supply (Ulfstrand 1967), but rather little is known about biotic factors influencing stream-dwelling invertebrate predators. Therefore, emphasis has been placed on this aspect of predator distribution. Responses of predators to the distribution of their prey are well known in other habitats, and could prove to be important as stabilizing forces in animal communities (Hassell & Rogers 1972; Hassell & May 1973). The stream is a headwater of the river Medway and is situated on the Ashdown Sands of Sussex (Nat. Grid ref. TQ 436327, altitude approx. 120 m). The study section, which is up to 1 m wide, 20 cm deep and with flow rates ranging from 0 to 25 cm s-1, runs through mixed woodland of mainly Alder (Alnus glutinosa Gaertner), with Oak (Quercus robur L.), Beech (Fagus sylvatica L.), Birch (Betulapendula Roth.), Holly (Ilex aquifolium L.) and Hazel (Corylus avellana L.). The substrate is variable, consisting of gravel, small stones and occasional rocks of up to 30 cm largest dimension and with extensive areas of fallen leaves and twigs. Except at the highest flow rates, the substrate is more or less covered with an ochre deposit of iron bacteria (probably mostly Leptothrix ochraceae Roth.) and ferric hydroxide derived from the iron bearing rocks of the area. The mean stream temperature during the study period was 13-3° C and the mean daily range 1-2° C (maximum daily range 2.0° C, minimum 0.5° C).