Two hundred and eighty‐seven longnose sucker Catostomus catostomus were collected from 14 lakes in Labrador, 52 from three lakes in Ontario, 43 from two lakes in British Columbia and 32 from a lake in Yukon; a total of 414 in all. The resulting 34 haplotypes (20 in Labrador) contained moderate haplotypic diversity ( h = 0·657) and relatively low nucleotide diversity ( π = 3·730 × 10 −3 . Mean ϕ ST (0·453, P C. catostomus popul...