OptiPrep gradients, as described for purifying viral isolates, may also be used for purifying marine viral assemblages. There are as yet, however, few descriptions of this application in the literature. CsCl gradients, on the other hand, have been used extensively for purifications of viral isolates and natural viral assemblages. Given the continued popularity of CsCl gradients, and their well-characterized performance, we present the following two-part purification and fractionation protocol for viruses in CsCl gradients. The first part involves relatively quick sedimentation through a step-gradient to remove the bulk of the contaminants. This is followed by a higher resolution continuous gradient to separate the viruses from residual contaminants and to separate viruses having differing buoyant densities from one another. We assume the starting material for the following procedure to be a concentrated suspension of viruses (whether filtered to remove cells or not).