Life-history characteristics such as growth, mortality, and size at sexual maturity of the seahorse Hippocampus reidi were investigated in situ in north-eastern Brazil from August 2006 to July 2007. The von Bertalanffy growth constant K was estimated at 1.195 year−1 and L∞ was set as 20 cm according to Taylor's relationship, using a Lmax of 19 cm. The smallest male with a brood pouch and the smallest male with a fully mature pouch measured 9 and 9.5 cm (height), respectively, while the smallest mature female measured 8.8 cm. Appropriate reproductive individuals were recorded throughout the study, with peaks from May to November. The size of first effective reproduction (carrying embryos) was 12.4 cm for males, a value higher than previously reported. Estimated instantaneous rate of natural mortality ranged from 1.43 to 1.58 year−1. The estimated life span for the species was 30 months.