The eggs of Coloburiscoides sp. (near haleuticus) were incubated in the laboratory at constant temperatures in 5-degrees-C intervals between 5-degrees-C and 30-degrees-C. There was a significant relationship between water temperature (T-degrees-C) and the length of the egg incubation period (Y days), expressed by the logarithmic equation: Y = 2217T-1.61 (r2 = 0.996, P < 0.01).Hatching success was high (> 80%) at temperatures between 10-degrees-C and 25-degrees-C. No eggs hatched either at 5-degrees-C or 30-degrees-C. The number of degree-days necessary for egg development decreased with increasing temperature. Eggs artificially fertilized in the laboratory had low (< 10%) hatching success.These results suggest that Coloburiscoides is warm adapted rather than cold adapted as previously thought, and suggest that its absence from Tasmania may be due to extinction during a recent glacial period. The low sychrony of life histories of Australian aquatic insects in lower altitude streams may be a response to higher temperatures rather than to hydrological variability.