Measurements on cultivated larvae and data from the literature demonstrate that length and width of pelagic sea lice larvae, Lepeophtheirus salmonis and Caligus elongatus, are overlapping. Therefore measurements on planktonic developmental stages are of little practical value in specific identification. A standard WP-2 net with 200-μm mesh size will, in practice, collect free living sea lice larvae satisfactorily. The naupliar and copepodid stages of sea lice are illustrated and pigment distribution described. Larvae of L. salmonis are characterized by black and brown pigment, those of C. elongatus by red. The distributions of pigment are distinctive in the species mentioned, and in pelagic larvae of Lepeophtheirus pollachius and L. pectoralis which also are measured, described and illustrated for comparison. The copepodid stages of larvae are different in shape of pro- and urosome, morphology and armature of the maxilliped, distribution of pigment and its colour. The black and brown pigmented copepodid of L. salmonis is slender and oval in outline, somewhat pointed anteriorly, broadest in the middle of prosome and more narrow posteriorly. The proximal segment of the maxilliped is robust without seta, but the subchela has a slender branching barb with three to five ‘fingers’. The red pigmented prosome of C. elongatus is elongate, broad anteriorly with two lateral notches. The extreme breadth of the copepodid is clearly before the middle of the prosome. The corpus of the maxilliped is elongate and equipped with a seta. In addition the appendage has a medial 2-divided process.