The neritic chaetognath Sagitta friderici Ritter-Zahony, 191 1 was identified in plankton samples from the northern Gulf of Mexico in June 1974. Specimens analyzed from two groups of adults ranged from 6 to 13.7 mm long. Meristic values did not appear to be a function of body length except for individuals in the 12.4 to 13.7 mm range that had larger numbers of hooks and teeth. Adults of sagitta friderici and the closely related S. tenuis Conant, 1896 were compared and found to be distinguishable chiefly by (1) the number of ova per unit length of the ovary, and (2) the arrangement of ova within the ovary. The TC values were highly variable and overlapping, thus casting doubt on the taxonomic importance of that characteristic. Sagitta friderici was abundant in inshore continental shelf waters where the salinity and temperature ranged from 24.9 to 33.9 ppt and 23.0 to 30.3’C, respectively. The failure to determine ecological boundaries between S. friderici and S tenuis revealed a need for more intensive sampling in coastal regions of the Gulf of Mexico.