Descriptions are given of all three larval instars ofCoelambus novemlineatus(Stephens, 1829) based on reared specimens from northern Sweden. A key to third‐instar larvae of six European species ofCoelambusis provided. Larva ofC. novemlineatusis very similar to that ofC. marklini(Gyllenhal), both being characterized by the presence of swimming hairs on tibiae and tarsi in combination with long cerci that only have seven primary setae. In addition, information is offered on the distribution and habitat of the species.