A new method for the analysis of behavior in small free-swimming aquatic organisms is described. In this procedure, called countercurrent separation, a dense solution flows down along the bottom of an inclined chamber while a light solution flows in the opposite direction, upward along the top of the chamber. The attraction of animals (injected into the center of the chamber) to one solution or the other is then determined by observing the proportion of animals that emerges from the chamber in that solution. When used with the nematode Caenorhabditis elegans, it is estimated that the apparatus is equivalent to at least nine theoretical plates.