Pale lateral bands that contrast with somatic colouration are common to many semi-aquatic spider species and may contribute to camouflage. Dolomedes plantarius is dimorphic for the presence or absence of a broad, pale, lateral band on the abdomen and cephalothorax. Here, we investigate the heritability of this banding pattern by assessing the proportion of banded progeny in broods of spiderlings for which the phenotype of one or both parents was known. Our results indicate a single-gene system of inheritance with the banded allele dominant to the unbanded. This finding offers a simple way to investigate various aspects of the biology of this rare spider, which is classified as vulnerable to extinction. We consider the implications for further understanding the mating system of D. plantarius and for studying the function and maintenance of banding in wild populations.