Relationships between the abundance of potential geosmin- and 2-methylisoborneol-producing (2-MIB) cyanobacteria and bacteria, and concentrations of the two taste and odour compounds (T&Os) were examined in a 7 day incubation of natural water from the surface and bottom of three reservoirs in southeast Queensland, Australia. Only a single known T&O-producing cyanobacterium (Geitlerinema spp.) was detected by microscopy at low density, and only in one reservoir. Densities of potential T&O-producing Streptomyces (determined by quantitative polymerase chain reaction (qPCR) assay) were highest in the bottom water and varied from 0.7 x 10(3) to 775 x 10(3) cells L-1. Geosmin ranged from 6 to 59 ng L-1 (with the highest concentrations in the bottom water), while 2-MIB varied from 6 to 47 ng L-1 (with the highest concentration in surface water). Concentrations of both compounds declined during the incubation under both light and dark conditions. Presence of the geosmin synthase gene, geoA, in cyanobacteria and Streptomyces was examined by different PCR approaches. Cloning of PCR products from amplification of geoA showed a high similarity to geoA in cyanobacteria, but not to streptomycetes. Our results demonstrate that more research on the ecology and molecular biology of T&O producers is required to better understand the dynamics of T&Os and to monitor emerging T&O episodes.