The mesostigmatid mite. Gammaridacariis brevisternalis has been found on both decomposing wrack and on the beach amphipods Orchcstoidea corniciilata and O. californiaiia. The percentage infestation of the host population increased with size of hosts and varied from 1.5 percent (hosts 3-7.9 mm) to 83.07 percent (hosts 16-19.9 mm). Mites showed no preference for male or female hosts. The number of mites per infested host increased slightly with amphipod size. Mites were found attached by gnathosonia exclusively on the ventral side of the host. The mites left hosts within 2-9 hours after death of the host and mites without hosts can crawl over sand at an average rate of 3 inches per minute. After leaving dead hosts, G. brevisternalis attach to new, living hosts. The mites traveled at least 30 cm in finding new hosts and observations suggest that the mites possess chemotactic .senses for locating hosts at a distance.