A pulse-purge schedule of exposure to labeled microorganisms was used to compare their digestibility by larval mosquitoes. Larvae were placed for an hour in suspensions of diverse axenically grown microorganisms that had been labeled with radioactive carbon (in the form of glucose or glycine). The guts of these mosquitoes were then purged with nonlabeled Sephadex particles for 30 min, and retained radioactivity was measured. Larvae imbibed no dissolved material. Larval mosquitoes differ in their capacity to derive label from algae (sensu lato), and certain algae contribute more label to these mosquitoes than do others. The nature of any algal food, as well as the feeding habits and developmental stage of the larva, influence its capacity to derive label from algae. This pulse-purge method of analysis can assist in the selection of algal "vectors" suitable as vehicles for transgenic larvicide. Although larval mosquitoes fail to assimilate the contents of Palmellacoccus cells with which they are confined, as much as 1/3 of the body contents of a Euglena gracilis cells become incorporated into their bodies. Because larval mosquitoes internalize more material from Euglena than they do from various other algae, these microorganisms provide a promising candidate vehicle for transgenic Bacillus thuringiensis israelensis.