Marine pathogenic bacteria and toxic phytoplankton species have significant impacts on coastal communities. Acute exposure to these deleterious microbes can result in outbreaks of communicable diseases, for example, cholerae, and poisonings through exposure to toxins. Interactions between phytoplankton and bacteria may influence phytoplankton bloom dynamics, toxin production by harmful algae and microbial species succession, all of which have downstream impacts ranging from human health to economic stability. A better understanding of the behaviors and interactions of marine pathogens with other organisms and with their environment could lead to more effective means for preventing the damaging effects these organisms can create. This chapter explores the patterns of microalgal and particle colonization by bacterial communities associated with two diatom species and the interactions of Vibrio cholerae with phytoplankton. A variety of methods were employed to visualize and quantify these relationships, including chemotaxis and motility assays to quantify the behavioral response of Vibrio cholerae to phytoplankton secondary metabolites. The larger role of particle colonization by marine bacteria as the gateway between microbes and humans is discussed, as is the role of phytoplankton health and physiology as a potentially important factor regulating the timing and nature of pathogen-phytoplankton relationships in the natural marine environment.