SummaryThe expression of the Mr30 000 surface antigen in the third stage larvae (L,3) ofBrugia pahangihas been investigated. The antigen could be detected only with great difficulty in the mosquito derived Li externally labelled with125I but was more easily labelled in 24 and 48 h post‐infective larvae harvested from the vertebrate host. Labelling of a detergent extract of mosquito derived L3with125I demonstrated that the Mr 30 000 antigen was indeed present in this life cycle stage, presumably in an internal localization. It seems likely that the Mr30 000 antigen is not fully expressed in the parasite cuticle until after infection of the vertebrate host. The data presented also suggest that there arc major differences in the surface properties of the mosquito derived Li compared to the p.i. L3harvested from the vertebrate hos