Three ariid fish, Netuma barba, Netuma planifrons and Genidens genidens, use the estuary of the Patos Lagoon (32ºS), Rio Grande do Sul, Brazil, as a nursery and feeding ground in various stages of their life history. A comparative study was made of the temporal and spatial distribution and relative abundance. Seasonal movements were interpreted on the basis of distribution and abundance of the young of these coexisting sea catfishe. Monthly otter trawl samples were made between November, 1979 and April, 1983. Temperature, salinities and dissolved oxygen contents were taken at each of the 674 sample stations. All three species were caught in ample temperature (12,1 to 26,3ºC), salinity (0,3 to 35,5‰) and dissolved oxygen (4,7 to 11,5 mg/l) ranges. However, they were more abundant in stations of low temperature and in salinity with highly dissolved oxygen content. N. barba was the most abundant catfish caught in the estuary, it constituted 81,9% in number and 81,4% in weight of ali ariid trawl catches during the study. N. barba occurred throughout the estuary, from the adjacent coastal region to the upper reaches of the Patos Lagoon estuary near the liminetic zone. N. planifrons occurred from adjacent coastal regions to low saline parts of the Patos Lagoon estuary, whereas G. genidens were distributed mainly in low saline and freshwater parts of the lagoon. Young-of-the-year N. barba aggregated near of the entrance of the lagoon in fall and winter, then moved up to intermediary parts of the estuary in spring, and they went to the upper part of the estuary in summer. Yearlings N. barba moved into the estuary in common with young-of-the-year but did not reach the coastal adjacent area. Young-of-the-year and yearling N. planifrons moved in to the estuary as the same pattern as N. barba but they became scarce during the late summer and early fall. G. genidens were scarcely captured in the estuary throughout the year. The spatial and temporal segregations among Netuma and Genidens were evident. A hypothetical model for the life history of N. barba is suggested.