The influence of exposure time upon macro-invertebrate colonization on modified Hester-Dendy substrate samplers was investigated over a 60-day period. The duration of exposure affected the number of individuals, taxa and community diversity.Tlie numbers of individuals colonizing the samplers reached a maximum after 39 days and then began to decrease, due to the emergence of adult insects. Coefficients of variation for the four replicate samples retrieved each sampling day fluctuated extensively throughout the study. No tendencies toward increasing or decreasing coefficients of variation were noted with increasing time of sampler exposure.The number of taxa colonizing the samplers increased throughout the study period-The community diversity index was calculated for each sampling day and this function tended to increase throughout the same period. Tliis supports the hypothesis thai an exposure period of 6 weeks, as recommended by the United States Environmental Protection Agency, may not always provide adequate opportunity for a truly representative community of macro-invertebrates to colonize multiplate samplers.Many of the taxa were collected in quite substantial proportions after periods of absence or extreme sparseness. This is attributed to the grov^^h of periphyton and the collection of other materials that created food and new habitats suitable for the colonization of new taxa.