ABSTRACT: During the mid-Pleistocene Transition (MPT), 2 families, 14 genera and 41 species of benthic foraminifera (Extinction Group) declined in abundance (8-12%of early Pleistocene benthic foraminiferal fauna) and finally disappeared at mid-bathyal depths in the Caribbean Sea (ODP Site 1000A, 916m depth), as part of the global extinction of at least 73 deep-water species at this time. At this site, the final phase of pulsed glacial decline, partial interglacial recoveries (0.8-0.67Ma), and final extinction (0.58 Ma) was essentially the same as the youngest level so far documented elsewhere (0.57Ma). ExtinctionGroup specimens had smaller average sizes during periods of decline than during favorable periods with higher abundances. Census counts on different size fractions indicate that this extinction event is best recorded by studies of shells in the 150-300µm size range. Pteropod dissolution proxies indicate that intermediatewaters in theCaribbean became less corrosive around the onset of theMPT (after c. 1.2Ma). This is interpreted to be a result of increased input of northern hemisphere-sourced intermediate water at the expense of southern-sourced Antarctic Intermediate Water. Intervals of enhanced Extinction Group decline in intermediate waters in the Caribbean occurred during glacials around the start (1.15-1.05 Ma) and end (0.83-0.65 Ma) of the MPT. During these glacial periods preservation of carbonate was optimal and ä13C values high (in sourcewaters at mid-depths in the NorthAtlantic), suggesting a causal linkwith enhanced inflow of a less-corrosive, colder, nutrient-depleted, well-ventilated water mass, such as Glacial North Atlantic Intermediate Water.