The zebra mussel (Dreissena Polymorpha) has been introduced into the Great Lakes and is expected to spread throughout much of North America. Monitoring for veligers is necessary in water treatment plants in order to determine when control strategies should be applied. In this case study, three years (1990, 1991, and 1992) of sampling results from a treatment plant that withdrew water from the Niagara River show that veliger densities varied considerably, with low densities (<10/L) found in >60 percent of samples. The sampling data for 1991 show that veligers and postveligers were present in the river from May to as late as December. On the basis of these data, strategies aimed at preventing veliger settlement may have to be applied for up to eight months per year. Includes 11 references, tables, figures.