A model^1,2, which explains the vertical and seasonal distribution of planktonic foraminifera was developed using data from the western North Atlantic and is comprised of three tenets: (1) The conditions in the photic zone (upper 80–100 m) exclusively dictate planktonic foraminifera species composition and abundance throughout the water column. (2) Species are vertically stratified within the photic zone according to their temperature preference. Depth distribution according to temperature preference has been proposed by several authors³ based on isotope analyses of sediments. Due to seasonal temperature variations, seasonal variations in the δ¹⁸O composition of seawater, and species-specific isotope fractionation, rigorous proof of this hypothesis cannot be provided from sediment analyses alone. (3) The chlorophyll maximum is a major food source zone, which is preferentially exploited by foraminifera. Our data from the Panama Basin, which are presented here, are confirmation of this model. The Panama Basin in the eastern equatorial Pacific, which has a shallow and steep thermocline, was sampled for living planktonic foraminifera. A multiple opening–closing net and environmental sensing system (MOCNESS)⁴ was used to collect 13 sets of eight vertically stratified oblique samples. We report here the quantitative distribution and stable isotopic composition of planktonic foraminifera from four MOCNESS tows taken over 5 days, during August 1979 (RV Knorr cruise 73) at 5°20′ N, 8°50′ W. Each tow consisted of eight samples which integrated ∼12.5-m intervals from 0 to 100 m for MOC 126, 25-m intervals from 0 to 200 m for MOC 127, 100–150-m intervals from 0 to 1,000 m on MOC 128 and 250-m intervals from 0 to 2,000 m on MOC 131.