New born guppies, Poecilia reticulata, were exposed to a sublethal concentration of nonylphenol (NP) for 90 days, with the aim of evaluating the influence that this pollutant has on reproductive functions. At the end of treatment, the gender balance was significantly biased towards females (sex ratio = 0.3 males per female). Treated males displayed anomalous transcription of vitellogenin in correlation with a significant reduction of the gonadosomatic index (GSI); however, an increase of vitellogenin gene transcription, correlating directly with an increase in GSI, was observed in NP treated females. The hepatosomatic index (HSI) was significantly increased in exposed males and females. Females exposed to NP showed no alteration of sexual behavior or reproduction. These findings in guppies suggest that NP has estrogenic potency sufficient to disturb reproduction. Effects were transitory in nature because three months in uncontaminated water allowed males to recover normal sexual efficiency.