Data are presented to indicate the nature of host-specificity shown by the juveniles of the isopod,Alitropus typus M. Edwards and the biochemical changes in certain host fishes attending the infestation. Even though specific host preference is apparently lacking, the frequency of attack on different fishes varies. Thus among the 9 species of fishes tried,Anabas testudimus (Bloch) was found to be most severely attacked by these isopods whereasPuntius sarana subnasutus (Val.) was comparatively less affected. Proximate analyses on 5 different species of fish, viz.,Channa orientalis (Cuvier),Anabas testudineus (Bloch),Lebistes reticulatus (Peters);Tilapia mossambica (Peters) andMacropodus cupanus (Cuv. and Val.) suggest depletion of biochemical constituents on account of infestation. An increase in the moisture content in the infested fish from those of the non-infested ones was noticed with the increase ranging from 1·19% in the femaleT. mossambica to 10·15% in the maleL. reticulatus. A drastic fall in protein, glycogen and lipid was also observed in the infested fish tested. Among the various fishes tried, the minimum depletion of glycogen occurred inC. orientalis (1·44%) and the maximum in the femaleM. cupanus (7·16%). In the case of lipid, the maleL. reticulatus showed only marginal reduction (0·96%) whereasC. orientalis showed 13·20% depletion. Regarding protein, the maximum reduction after infestation was observed inA. testudineus (20·91%). The possible reasons for the depletion of these biochemical constituents in the infested fish are discussed in detail.