The production of volatile odour compounds by freshwater phytoplankton was monitored weekly from November to April (summer period) 1990/91 at two sites: (1) Hay Weir pool on the Murrumbidgee River, NSW and (2) Carcoar Dam, near Blayney, NSW. During this period, the phytoplankton of the Murrumbidgee River was dominated by two species of the diatom Melosira, and the cyanobacterium Anabaena sp. Carcoar Dam was mostly dominated by the cyanobacteria Microcystis aeruginosa and Anabaena sp. The major odour compounds detected were geosmin, β-cyclocitral, β-ionone, geranylacetone, and 6-methylhept-5-en-2-one. Clmparison of multivariate statistical analyses of the volatile odour compound profiles and algal population data provided strong evidence for the hypothesis that the major source of many of these odour compounds was the phytoplankton. Total (intra+extracellular) geosmin concentration was strongly correlated with Anabaena abundance with no significant difference in geosmin production between sites. From the overall average of 10 fg geosmin cell−1 it is possible to predict that taste and odour problems, due to geosmin, may be experienced at Anabaena abundances of > 1,000-2,000 cells ml−1 in temperate Australian waters. β-cyclocitral concentration was correlated with Microcystis abundance at Carcoar Dam (10 fg β-cyclocitral cell−1), but with Anabaena sp. abundance at Hay Weir (2 fg cell−1).