In 2001, Helike Project archaeologists found traces of the coastal site of Classical Helike, a city destroyed and submerged by an earthquake and tsunami in 373 BC. Nearby they also found remains of its prehistoric predecessor, an Early Bronze Age (EBA) settlement dating from ca. 2500–2300 BC. Both sites are on land, 3 to 5 m below the surface of a coastal fan delta southeast of Aigion, on the Gulf of Corinth in Greece. To investigate the environmental history of the delta and the ancient sites, we analyzed microfossils, mainly ostracods and foraminifers, in sediments from boreholes and archaeological trenches. Although the area is now dry land, microfossil assemblages indicate ancient coastal aquatic environments over a wide range of salinity at the time and place of sediment deposition. The EBA site is covered by clay and sandy clay containing freshwater, brackish, and marine microfossils. The Classical site is covered by silty sand and silty clay containing mostly freshwater and brackish microfossils. The EBA site was apparently submerged in a coastal lagoon, perhaps due to an earthquake. The site was subsequently silted over and uplifted. Some two thousand years after the EBA destruction, the historical earthquake and tsunami destroyed Classical Helike and submerged its ruins in a lagoon. Continued uplift and sedimentation then led to the emergence of the present delta. The occurrence of severely abraded or fragmented shells of marine and brackish microfossils in a broad stratum of possible Classical age suggests the action of a tsunami.