To increase the accuracy of surveys of epilithic invertebrates like zebra mussels, we recommend a new technique for estimating density in cobble and other coarse substrate areas. The density of benthic macroinvertebrates per unit of bottom area (BMI/BA) is estimated by multiplying individual observations of the number of organisms per unit of rock surface area (BMI/RSA) by the rock surface area per unit of rock-covered bottom area (RSA/RBA=3.4 in our study area) and the proportion of rock cover in the area (RBA/BA= 0 to 1.0).