This report describes the characteristics of estrus in the female collared peccary (Tayassu tajacu) from the eastern Amazon. Two consecutive experiments were performed. The objective of the first experiment was to assess the efficacy of estrus diagnosis by serum estradiol-17β measurements, vaginal cytology and vulval appearance. The second experiment was designed to evaluate the effectiveness of these features for predicting the period of mating acceptance. In Experiment 1, we monitored 56 presumptive estruses in 14 captive females. Estrus was characterized by the reddish and tumescent appearance of the vulva, the presence of vaginal mucus and vaginal opening, which persisted for 4, 4, 2, and 2 days, respectively. In vaginal smears, superficial plus intermediate cells became predominant (accounting for 60% of all cells) for a 4-day period, from 2 days before the estradiol-17β peak. Superficial cells did not attain their maximum density (45% of all cells) until approximately 1 day before the serum estradiol-17β peak. Following estrus, superficial and intermediate cells were notably reduced and the appearance of the vulval appearance was characterized by a pale, non-tumescent vulva and vaginal closure. A complete concordance in estrus diagnosis by the 3 methodologies was observed in 35 (62.5%) of the total 56 estrus predictions. In Experiment 2, mating was confirmed in 10 out of the 14 cyclic females. Sexual receptivity was observed in the last third of the period during which secondary estrus signs were shown. In conclusion, examining the vulval appearance and taking a vaginal smear constitutes a useful method of detecting estrus and predicting the period of male acceptance of the collared peccary female. Estrus was indicated by a predominance of superficial cells, and a proportion of superficial plus intermediate cells higher than 60% in the vaginal smear. © 2006 Elsevier B.V. All rights reserved.