The present work analyzes the relationship between length and weight for 28 fish species belonging to 16 families in Celestun lagoon, a natural marine reserve located in southeast Mexico that is influenced by freshwater seeps. With interrupted periods, a total of 14 846 specimens were collected from 1990 to 2006. The most abundant species were Eucinostomus argenteus, Eucinostomus gula, Anchoa mitchilli and Lagodon rhomboides. Exponents varied around b = 3.17 at P < 0.001, indicating a preference for isometric relationships. This study presents the first estimation of LWR for four species (Menidia colei, Lucania parva, Floridichthys polyommus, Garmanella pulchra) and the maximum length record for F. polyommus (19.5 cm standard length) that is greater than previously recorded.