The La Plata croaker (Pachyurus bonariensis) family Sciaenidae, is a small (< 23 cm) benthopelagic fish that originated in the Parana- Paraguay-Uruguay river systems in South America. Its first record in the Patos Lagoon drainage area, Rio Grande do Sul (Brazil) occurred in 2001. This research reports for the first time, the occurrence of both juvenile and adult specimens of P. bonariensis in the Sao Goncalo Channel (a 75 km long waterway between Patos and Mirim lagoon) and in the Mirim Lagoon, in 2005 and 2007 respectively. A total of 434 gonads were analyzed histologically from specimens captured by artisanal fishermen at Capilha, Mirim Lagoon between 2008 and 2010. Gonadal development was observed and described. Pachyurus bonariensis probably dispersed from northern Patos Lagoon in 2001 to the Sao Goncalo Channel by 2005. The occurrence of adults in limnic waters in the Sao Goncalo Channel and further south in the Mirim Lagoon in 2007, confirm that P. bonariensis is now an established species in Mirim Lagoon and could become a commercially important species to the artisanal fishermen of the area. Likewise at the limnetic northern parts of Patos Lagoon, P. bonariensis is now abundant at Mirin Lagoon and the ecological impacts of this range extension must be investigated.