Abstract: A line-transect survey conducted in February 2004 along 780 km of trackline in the outer Sundarbans Delta of Bangladesh resulted in abundance estimates of 1,382 (CV=54.8%) and 5,383 (CV=39.5) for finless porpoises and Irrawaddy dolphins, respectively. These estimates indicate that the waters of Bangladesh support regionally important populations of both species. The optimistic conservation implications of the survey are tempered by observations of potential unsustainable bycatch in gill net fisheries targeting elasmobranchs. One-way ANOVAs (df = 1, 84) indicated significant differences between the two species for depth (Prob. = 0.001), temperature (Prob. 0.004), salinity (Prob. 0.001) and probably turbidity (Prob. = 0.057). A discriminant analysis pointed towards depth and salinity as the two most important factors that differentiate environmental preferences of the two species. A line transect survey conducted in February/March 2005 along 955 km of trackline in the Mergui Archipelago of Myanmar recorded only two sightings of finless porpoises (one of five and one of two individuals) and one sighting of Irrawaddy dolphins (12 individuals). All three sightings were within 15 km of each other in a small semi- enclosed bay offshore of the Kyaukpa and Tenasserim river mouths and their associated mangrove forests. The paucity of sightings and limited range of finless porpoises and Irrawaddy dolphins recorded during this survey suggest that both species occur only in low densities in the Mergui Archipelago, probably due to the lack of large freshwater inputs and possibly incidental catches in the intensive gillnet fisheries that operate in the area where the animals were observed.