Estrogens are implicated in male gonad function, although their physiological roles remain uncertain. In the present study, we take advantage of the original model of spatio-temporal organization of trout spermatogenesis to revisit the synthesis and action sites of estrogens in fish testis. Within this system, somatic cell and germ cell development are synchronized due to a strict seasonal spermatogenetic cycle and the cystic organization of gonads. We evaluated the expression patterns and regulation of 3 aromatase isoforms (cyp19a, cyp19b-I, and cyp19b-II) and 4 estrogen receptors (esr1a, esr1b, esr2a, and esr2b) by quantitative reverse-transcriptase PCR during testicular maturation and in isolated germ cell populations. Our data demonstrated a reciprocal relationship between cyp19a and cyp19b (I and II) expression during testicular development (cyp19a decreased while cyp19b increased with maturation). Furthermore, cyp19b is significantly expressed in late germ cells. At the protein level, aromatase was immunohistochemically identified in interstitial tissue and in germ cells. Remarkable elevation of esr1a and esr2a was observed during the final stage of spermiation, while esr1b was expressed in an early stage of spermatogenetic development. Estrogen implants reduced testicular cyp19a transcript abundance while up-regulating cyp19b levels whereas androgens up-regulated testicular esr1a, esr2a, and esr2b. Together, the distinct spatio-temporal expression profiles and regulation of aromatases and estrogen receptors suggest that estrogens have discrete physiological functions during an early step of spermatogenesis and in the final stages of germ-cell maturation and/or excretion. This article is protected by copyright. All rights reserved.