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Densities of planktonic larvae of the Asian freshwater invasive pest bivalve Limnoperna fortunei (Dunker) were monitored weekly between 19 September 1997 and 22 January 1999 at two sites: the Paraná de las Palmas river, ca. 130 km north of Buenos Aires (33°57.5′ S, 59°12.5′ W), and a coastal station on the Río de la Plata estuary, in Buenos Aires (34°33.5′ S, 58°24.5′ W). Very variable but generally high (up to 25 000–35 000 ind. m⁻³) concentrations were recorded at both sites in September 1997-April 1998, and again in August 1998–January 1999, suggesting continuous reproduction during these months. In May through July 1998 larval densities were generally below 100-200 ind. m⁻³. The threshold temperature level associated with the onset and termination of reproduction was ca. 16–17 °C. With the exception of August-September 1998, when Río de la Plata temperatures were below …