Zooplankton (predominantly calanoids and cyclopoids) from the Caribbean were analyzed for 238U, 234U, 232Th, 228Th, 228Ra, 226Ra, 210Pb, and 210Po. The concentration factor in the zooplankton relative to seawater is highest for 210Po. 210Pb and 228Th have similar concentration factors and are comparable (when normalized to 226Ra) to reported fiber‐scavenging experimental data on 210Pb and 234Th. The dominant transport agent for 210Po and 210Pb from the mixed layer to depth cannot be unmodified zooplankton debris.