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Laboratory strains of Xenopsylla cheopis from La Perla, Puerto Rico, and San Francisco, California, were tolerant to malathion (adult LC50, 0.9–1.9%) and tolerant to fenthion (0.19–0.33%) in tests with World Health Organization test papers. Test papers prepared in our laboratory showed that Abate and Baygon were not significantly effective against adults or larvae. Coumaphos, effective against adults, did poorly against larvae. Naled and carbaryl gave best results against both larvae and adults but because the papers were prepared differently it cannot be claimed that either one is better than fenthion.