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The microdistribution of stream insects in relation to current velocity, substratum particle size, silt, and detritus was studied in field experiments utilizing substratum-filled trays. 31 % more organisms occurred in trays placed in a riffle than in those placed in a pool. In both riffle and pool the amount of colonization for most taxa was least on the smallest substratum sizes studied (0.5-0.7 cm diam.), was greater on the 1.0-2.0 cm size, reached a maximum on the 2.5-3.5 cm size, and was markedly reduced on the largest substratum size (4.5 x 7.0 cm). The reduction of current velocity alone accounted for reductions in the numbers of four or five species, while the addition of a light coating of silt (< 1 mm deep) significantly reduced the numbers of only three species. The 1.0-2.0 cm substratum trays consistently contained more small-sized (<3.95 mm) detritus particles than did the trays filled with the largest sized substratum. When the amount of these particles was similar in both sizes of substrata, the preference previously shown by the insects for the small substratum did not hold. Thus, insects may colonize small (1.0-3.5 cm) substrata primarily because these serve as a better food collecting device than do larger (or smaller) substrata, and manipulation of the substratum may alter the productivity of a stream through an influence on its detritus storage capability. The substratum-detritus interaction was the overriding influence on insect microdistribution under the conditions of this study and current velocity and a light deposition of silt play only secondary roles. A general model was formulated to show the interaction of biotic and abiotic factors which influence the microdistribution of stream insects.