Bacteria belonging to the Streptomyces genus are known to produce several taste-and-odour compounds (TOCs), but knowledge on the abundance of streptomycetes in drinking water reservoirs and other aquatic environments is scarce. In this study, quantitative real-time polymerase chain reaction (qPCR) was applied, for the first time, to determine densities of streptomycetes in a river, at a weir and in two reservoirs in subtropical Australia. The PCR approach was optimized with respect to (a) collection of streptomycetes in water, (b) extraction of DNA, and (c) a procedure to correct for inhibition of PCR amplification by natural substances in the water. Mean densities of Streptomyces cells at the study sites varied from 225 to 45,650 cells L−1. The highest density occurred in bottom water (8.5 m deep) of one of the reservoirs, while densities in the Brisbane River varied between 260 and 7,950 cells L−1. At the weir site, seasonal variation in abundance in winter and spring in surface water (mean densities of 430–13,550 cells L−1) did not correlate with total bacterial abundance (0.9–3.5 × 109 cells L−1). The qPCR approach shows that quantitation of streptomycetes in fresh water can be successfully achieved and may prove valuable in predicting TOC episodes in aquatic systems used for drinking water supplies.