The effect of the size of the surface area of a fish host on settlement and initial survival of Lepeophtheirus salmonis (Krøyer, 1837) was determined. Atlantic salmon Salmo salar L. of various sizes-small (43 +/- 4 g), medium (173 +/- 32 g) and large (644 +/- 62 g)-were initially examined to ascertain their respective body surface area, excluding gill surface. The initial examination showed a size-dependent body to fin area ratio, with the fin area of small fish representing 34% of the total body area and the fin area of medium and large fish representing 26 and 23%, respectively. Regression analysis of the body weight and standard length against total body surface area gave a good correlation and high R2 values. Two simultaneous experimental infections with approximately 7,000 copepodids of L. salmonis were carried out on fish populations of mixed sizes consisting of 30 small fish, 10 medium-sized fish and 5 large fish, with an approximate total surface area per size group of 2700 cm2. Higher numbers of parasites were found on the small size group, which also had the highest parasite density, with 0.25 and 0.45 parasites cm(-2). Comparison of samples of 5 fish per experimental group revealed that the larger fish had the highest mean numbers of parasites, but the smaller fish still retained the highest density. There was a statistically significantly higher settlement on the fins than on the remainder of the body surface in all size groups. Highest numbers of parasites were particularly found on the dorsal and pectoral fins. Examination of the surface surface area revealed that the dorsal fin had the greater parasite density, with >2 parasites cm(-2) in all fish size groups. These data provide insight into the effects of the amount of host surface area available on parasite settlement and survival, and highlight the potentially increased susceptibility of farmed salmon smolts to infection of this ectoparasite.