Following a method of rearing the tick parasite, Hunterellus hookeri, which is described, 90,000 [female][female], bred from a Texas strain, were released in 2 localities on Martha''s Vineyard, Mass., in 1937, 1938, and 1939, in an attempt to control the American dog tick, Dermacentor variabilis. No parasites were recovered from 1937 to 1942 and no reduction in tick abundance attributable to the parasite was observed. No parasites survived the winter in exptl. rearing. Also, ticks on mice within 10 ft. of liberated emerging parasites were not parasitized. A strain of H. hookeri from France, released by Larrouse and others on the Elizabeth Islands in 1926, has apparently persisted as one parasitized nymph of Ixodes scapularis was collected there in 1941. Ixodiphagus texanus, an indigenous sp., was found in larvae and nymphs of D. variabilis on the Elizabeth Is., but not on Martha''s Vineyard, where both were also present.