Zooplankton and fish larvae in a Portuguese estuary were studied from 2003 to 2004. Different nets were used to catch them at different tides. The size of the net affected the abundance of zooplankton. Freshwater crustaceans were found in the upper part of the estuary, while marine copepods were more common in the lower part. The most abundant fish larvae were from resident species, and their numbers increased with water temperature. Salinity and temperature were the main factors affecting where zooplankton were found. Human activities also influenced the abundance of these tiny creatures in the estuary.