The effect of substrate type and potential refuge on the vulnerability of four stream-living insect prey to two insect predators was studied by compring predator capture success on each substrate in laboratory stream chambers. Prey were two Ephemeroptera, Baetis tricaudatus (Baetidae) and Ephemerella subvaria (Ephemerellidae), Simulium vittatum (Diptera:Simuliidae), and several species of Hydropsychidae (Trichoptera). Single prey taxa or mixed prey species were exposed to the predators Agentina capitata (Plecoptera: Perlidae) and Nigronia serricornisa (Meqaloptera: Corydalidae). Substrate treatments were: very coarse sand, gravel-pebble, artifical turf, and, when testing for the protectiveness of hydropsychid retreats, gravel with additional sand. In single and mixed-prey experiments, mayfiles, especially Baetis were more vulnerable than other prey. When given no choice of prey, both predators captured prey on all substrates. Substrate significantly affected the vulnerability of three taxa, but in different ways for different predators. Turf allowed the caputure of more Baetis by Nigronia, but fewer Baetis and Ephemerella by Agnetia. On gravel, hydrosychids were vulnerable to Agentina; additional sand, however, allowed the construction of more or stronger retreats, reducing vulnerability. The same protection resulted from a 24-hour retreat-building period before exposure to Agentina. With a choice of prey, Nigronia captured Baetis on all substrates, Ephemerella and Simulium on turf and gravel, and hydropsychids only on gravel. Agentina with mixed prey took specifically more Baetis on all substrates but did not catch Ephemerella on turf. The swimming of Baetis probably contributed to its high vulnerability; the crawler (Ephemerella) and clingers (Simulium, hydropsychids) were less conspicuous, or, in the case of Simulium, out of reach on the walls or ceilings of the chambers. The two predators also differed in their ability to forage on turf and sand. Agnetina was less successful in catching prey on turf whereas Nigronia had no difficulty. On sand, however, Agentina was just as effective as on gravel whereas Nigronia caputured only swimming prey on sand when a choice was available. These contrasting results are attributed to differences in the predators'' antennae and agility. Therefore, substrate influences vulnerability not only by providing direct shelter but also by affecting the ease with which a predator can sense or pursue prey on particular substrates.