Spermatogenesis and ecdysteroids were studied in male-destined fourth instar nymphs of the tickOrnithodoros parkeri. Feeding stimulated spermatogenesis. Elongated spermatids appeared well before epicuticle deposition and continued to be formed until after ecdysis. Ecdysteroid titers were measured by RIA in individual ticks that were categorized according to the stage of spermatogenesis. The titer was low at the spermatogonial stage, increased as the number of primary spermatocytes increased, and reached a small peak at the time when many cells reached late prophase of meiotic division I. HPLC-RIA analysis tentatively demonstrated the presence of only 20-hydroxyecdysone and ecdysone at that time. Then the titer dropped to a low level as soon as the elongated spermatids appeared at the distal pan of the testis. A second and larger peak of ecdysteroids appeared during the continuous formation of the elongated spermatids. However, this peak was obviously correlated with epicuticle deposition.