We have described the results of laboratory analyses of 42 horizontal tows using a Clarke-Bumpus plankton sampler with a 100 µ emmesh net in the Gulf Stream. Classifications of 20 zooplankton taxa have been made for the 42 hauls. Calanoids dominate the hauls. It was found that the zooplankton population distribution and density varied considerably from haul to haul, but there was no consistent buildup or depletion of population with progress of the flow north in the Gulf Stream. These biological arguments reinforce previous conjectures that there is a geostrophically-balanced inflow of continental shelf and Labrador Sea water that gradually mixes with the Gulf Stream water mass. These inflows provide sources of zooplankton and phytoplankton to the Gulf Stream as it progresses northward. Also, an ecological analysis of the Gulf Stream indicates that the dry organic matter is over an order of magnitude below that which is theoretically possible.