Growth and sexual maturation of Astyanax fasciatus were studied in shallow areas of the Arenal reservoir in Costa Rica. Von Bertalanffy's equation was T1 = 167.3 (1 - e(-0.093 t - 0.132)). The mean growth for the first six months was 11.7 mm/month, 6.9 for the six following months, and diminishing to 4.0 mm/month in the following months. The relationship between total length and weight was: W = 8.6 × 10-6 Tl3.081 for males and W = 1.8 × 10-6 Tl3.434 for females. Males and females attain sexual maturity at Tl between 70 - 80 mm, approximately when they are six months old. The species reproduces all year with peak activity between July and September.