Technical Abstract: We used an expressed sequence tag approach to study the genome of the southern cattle tick, Boophilus microplus. A normalized cDNA library was synthesized from pooled RNA purified from tick larvae which had been subjected to different treatments, including acaricide exposure, heat shock, desiccation, cold shock, host odor, and infection with Babesia bovis. For the acaricide exposure experiments, we used several strains of ticks which varied in their levels of susceptibility to pyrethroid, organophosphate and amitraz. We also included RNA purified from samples of different developmental stages and tick organs. Template was prepared from 11,520 clones and both 5' and 3' EST data was generated from each clone. The sequence data was used to search public protein databases and a B. microplus gene index was constructed, consisting of 8,270 unique sequences and their associated putative functional assignments, when available, which can be viewed at the TIGR website (http://www.tigr.org/tdb/tgi). A number of ESTs were identified which possessed significant sequence similarity to genes which might be involved in resistance to acaricides.