El plancton, es un término colectivo utilizado para denominar a una serie de organismos marinos y dulceacuícolas que van a la deriva o que flotan en la superficie del agua. Este se divide en fitoplancton y zooplancton. El fitoplancton se puede encontrar tanto en el medio marino como en dulceacuícola. En el mar, los grupos más sobresalientes son las diatomeas y los dinoflagelados, responsables de la productividad primaria, por lo cual son los iniciadores de la cadena trófica en ese medio (Acleto, 1998) Phytoplankton community species composition and abundance over time in Acapulco Bay, Mexico, were quantified from September 2009- April 2010, JanuaryApril and September 2011 to April-July and October and November 2012. Phytoplankton was taken at 2 stations, morro San Lorenzo and casa Díaz Ordaz. A total of 99 species were identified: 50 dinoflagellates, 40 diatoms, 4 cyanobacterias, 2 silicoflagellates, 2 chlorophytas and 1 charophyta. The dinoflagellate dominated during the rainy season and the diatoms dominated during the dry season. At least 7 potentially toxic dinoflagellate species were recorded which could generate harmful algal blooms (HABs) if bay environmental conditions are altered. In April 2012 an AB of Neoceratium balechii nontoxic species was recorded. The phytoplankton community proved to be a good indicator of environmental change in the Bay of Acapulco and its continuous monitoring is recommended as a way to detect changes in environmental quality of the bay