Sea lice ( Lepeophtheirus salmonis ) infection negatively affected swimming performance and postswim body ion concentrations of juvenile pink salmon ( Oncorhynchus gorbuscha ) at a 0.34 g average body mass but not at 1.1 g. Maximum swimming velocity (U max ) was measured on over 350 individual pink salmon (0.2-3.0 g), two-thirds of which had a sea lice infection varying in intensity (one to three sea lice per fish) and life stage (chalimus 1 to preadult). For fish averaging 0.34 g (caught in a nearby river free of sea lice and transferred to seawater before being experimentally infected), the significant reduction in U max was dependent on sea lice life stage, not intensity, and U max decreased only after the chalimus 2 life stage. Experimental infections also significantly elevated postswim whole body concentrations of sodium (by 23%-28%) and chloride (by 22%-32%), but independent of sea lice developmental stage or infection intensity. For fish averaging 1.1 g (captured in seawater with existing sea lice), the presence of sea lice had no significant effect on either U max or postswim whole body ions. Thus, a single L. salmonis impacted swimming performance and postswim whole body ions of only the smallest pink salmon and with a sea louse stage of chalimus 3 or greater.