The scanning electron microscopical appearances of infective larvae of Strongyloides ratti and Ancylostoma caninum have been described. Particularly noteworthy was the tail of S. ratti which was found to have a distal aperture surrounded by a row of eight projections. The penetration of larvae through newborn mouse skin was investigated. S. ratti larvae forced its way rapidly and directly through the stratum corneum. No larvae of A. caninum were observed in stages of partial penetration but the occasional empty sheath was seen.