EN
The experiments made on the laboratory mice and rats demonstrated the existence of differences in the pathogenicity among the 3 T. spiralis strains: laboratory one, freshly isolated from man in Poland, and that derived from Kenya. The differences were accounted for by a varying degree of infectivity among the strains (number of muscular larvae after the same infecting dosis), which showed an increasing tendency in the subsequent passages through the same host species. We are grateful to Dr. G. S. Nelson for supplying us with the Kenya strain and early information about his results (Nelson and Mukundi, 1963).