A polymorphism in the enzyme malate dehydrogenase in Dorset populations of Myrmica rubra was detected using isoelectric focusing (IEF). The polymorphism was not detected on native polyacrylamide gels. Two forms, with pl values of 4.9 and 5.7, were resolved. Several lines of evidence show that the polymorphism has an environmental rather than a genetic basis. The cause of the change from one phenotype to the other may be related to a seasonally varying factor. The results indicate that whilst IEF has great potential for revealing isozyme polymorphisms in ants, care should be taken in interpreting results.