The objective of the current study is to understand population characteristics of the grass shrimp Palaemonetes pugio by describing growth, population structure, mortality, fecundity, and size at sexual maturity from a lagoon system inlet in the southwestern Gulf of Mexico. The shrimps were collected between January and December 2007 by otter trawl of 1-mm mesh size; salinity and water temperature were recorded simultaneously. Regressions were applied between total length (TL) and total weight (TW) of females and males. Growth parameters were estimated by length-frequency distributions with 0.01-mm length interval of carapace using the von Bertalanffy growth function (VBGF), mortality was calculated with an exponential model. To represent fecundity, the potential model was selected. The average salinity and temperature values were 6.39 ± 2.49 ppt and 27.63 ± 1.59°C, respectively. The number of females was higher than for males in winter and spring, suggesting a reproductive strategy that increases the probability for the male of finding a receptive female. The K (Bertalanffy growth coefficient) values were 0.48 for males and 0.43 for females. This could indicate a reduction of female energy investment in growth, directing it rather to reproduction. The instantaneous mortality estimates (Z) suggest a relationship with the reproductive period; the highest mortality was encountered from April to September. Female size at sexual maturity was estimated to be 2.41 cm TL, showing that ovarian development starts in winter and continues until early spring. Consequently, P. pugio showed faster growth, a seasonally oscillating growth pattern, a rather short life span, a somewhat high mortality, and an early onset of sexual maturity, which is considered to be similar to r selected species.