1. We examined fluctuating asymmetry (FA) among individuals of the stream-dwelling stonefly Nemoura trispinosa (Plecoptera: Nemouridae) to determine whether individuals exposed to an increase of 2–3•5 °C in water temperature would show greater FA than reference (control) individuals. 2. Mature nymphs were collected from two adjacent channels (one experimental, one control) in a longitudinally divided stream both before and during a 2-year temperature manipulation. No consistent differences were found between the experimental and control channels for any measure of FA. 3. Four additional reference sites were studied to estimate ‘natural’ variation in FA, and to assess any relationship between FA and population genetic structure (e.g. heterozygosity). Variation in FA among these sites was greater than that resulting from the manipulation. Allozyme analysis indicated low to moderate levels of genetic differentiation among sites (Wright’s FST mean = 0•06, maximum = 0•13) and there were negative correlations between FA and heterozygosity (Hexp). 4. We surveyed experimental studies published since 1996 to evaluate the generality of our results. Of 44 comparisons examining an association between an experimental stress and FA, 19 (43•2%) failed to detect any relationship. This pattern did not depend on the taxonomic group or the number or type of traits, although some stressors appeared to be more likely to produce an increase in FA than others. 5. We conclude that FA may be unreliable for detecting subtle biological changes resulting from small temperature shifts, and concur with others that the technique should be viewed with extreme caution as a monitoring tool.