Abstract Beam attenuation data from a Sea Tech 1-m transmissometer at 190 CTD stations in the northwest Atlantic in 1982 were collected to determine the spatial distributions of particulate matter in warm core rings of the Gulf Stream. Plots of beam attenuation coefficient, C, vs depth showed consistent offsets between down- and up-cast data in parts of the water column with pronounced temperature gradients. A simple correction scheme removed this bias from Sea Tech data. A regression of suspended particle matter (SPM) concentration, determined by large volume in-situ filtration sampling, vs beam attenuation coefficient gave a C value of 0.358 ± 0.003 for particle-free water for this instrument. SPM calibrations of beam attenuation coefficient for the slope water, warm core rings, the Gulf Stream and Sargasso Sea have been determined.