Numerous studies have been conducted on ballast water species composition and diversity in other countries but not in the Philippines. Thus, this study aimed to provide baseline information on the plankton diversity in ballast water of the inter-island passenger-cargo vessel calling the ports of Iloilo-Bacolod-Manila- Cagayan de Oro, Philippines and vice-versa. Specifically, this study aimed to determine the presence of phytoplankton and zooplankton diversity and species density of this plankton measured in cells/ml using the haemacytometer technique. Composite sampling was employed having one liter of ballast water used per ballast tank. A total number of 15 genera of phytoplankton (diatom) and one genus of zooplankton were recorded. Chroococcus, Nannochloris and Protococcus had the highest cells/ml while Ankistrodesmus, Micromonas and Synedra had the lowest cells/ml. The most common phytoplankton observed in ballast tanks were Nannochloris and Protococcus. Neocalanus (copepod) was present in all ballast tanks except in ballast tank 1 (fore-peak). The phytoplankton and zooplankton composition was found to be non-invasive in nature showing its ubiquity in the marine environment. This study provides an initial assessment or preliminary list of phytoplankton and zooplankton diversity from the ballast water of a passenger-cargo vessel calling the Philippine ports.