In Callianassa louisianensis, ovarian development becomes evident in December and con tinues through the spring. Ovigerous females occur primarily from early June through August, less frequently into September. Recruitment occurs throughout the summer and early fall, and juveniles reach large enough size to be detected in our samples by early spring. First-year females enter the breeding population the following summer; the smallest ovigerous among these have attained a carapace length (CL) of almost 11 mm. Analysis of relative growth suggests that females enter a maturation growth phase at about 11 mm CL, while males enter a maturation phase at near 15.5 mm CL. In males, this transition is marked by a strong increase in the relative growth of the major chela, together with a change in shape of the chela. In females, there is a decrease in the relative growth of the major chela after maturation. Carapace lengths in females do not attain the size of those in the largest males. In the maturation phase, female growth appears directed toward lengthening of the abdomen and development of ovaries, while in males it is directed toward development of the major chela, a secondary sexual structure that may be utilized in aggressive encounters. The ghost shrimp Callianassa louisianen sis (treated formerly as Callianassa jamaicense var. louisianensis Schmitt, 1935) is a widespread and abundant component of the estuarine macroinvertebrate assemblage in the northern Gulf of Mexico (Willis, 1942; Hedgpeth, 1950; Wass, 1955; Phillips, 1971; Felder, 1973). Concentrated in intertidal and shallow subtidal substrates ranging from sandy mud to organic silty sand, this fossorial species is adapted to oligohaline hab itats of coastal marshes, tidal channels, and estuarine embayments. Its range extends from western Florida through the Gulf states and southward at least to the Rio Carrizal estuary in Tamaulipas, Mexico. Osmoregu latory adaptations of adults and larvae have been documented (Felder, 1978; Felder et al., 1986), as have adaptations of the adults to periodic anoxia in their burrows (Felder, 1979). However, given the difficulty of monitoring natural populations in their fossorial habitats, little is known of growth, maturation, and reproduction in the species. The dominance of this species among the estuarine infauna, its role in marine food chains, its impact on plant and animal com munities, and its contribution to sediment and nutrient turnover likewise have re ceived little attention, though some of these qualities have been evaluated in popula tions of other western Atlantic thalassinoids