Earthworms and microarthropods are abundant in turfgrass and may be important to thatch decomposition and nutrient recycling. There is little published information concerning the effects of cultural practices on the soil fauna in turf. The purpose of this study was to determine the influence of prolonged N fertilization on populations of earthworms, Collembola, and Acari involved h decomposition processes in Kentucky bluegrass, and to relate these effects to changes in soil pH and thatch accumulation. Six rates of ammonium nitrate fertilizer ranging from 0 to 25 g N m−2 were applied annually for 7 years to replicated plots of ‘Kenblue’ Kentucky bluegrass (Poa pratensis L.) growing on a Maury silt loam soil (fine‐silty, mixed, mesic Typic Paleudalf) near Lexington, KY. Populations of earthworms and soil microarthropods were enumerated in May and October 1983. Increasing the rate of N fertilization resulted in a significant decline in soil and thatch pH and in exchangeable Ca and K, and caused a significant increase in thatch. Regression analyses indicated a highly significant linear decrease in earthworm density and biomass as annual rates of N fertilization increased. Collembola were more abundant at an intermediate fertilizer rate, whereas Acaridae were unaffected by N fertilization. Cryptostigmata, predominantly Scheloribates sp., were the most abundant arthropod decomposers in the turf. Each of the seven oribatid mite species differed in its response to N fertilization. The study indicates that when N fertilizer is applied to Kentucky bluegrass at rates sufficient to cause soil acidification, populations of earthworms and certain other decomposers may be severely reduced. Thatch accumulation was negatively correlated With earthworm density and biomass, although other factors probably also contributed to thatch development.