The migration of species from newly established communities to previously uninhabited islands was studied in laboratory systems during early phases of colonization using source pools of protozoan species from natural systems. Assemblages of protozoan species to be used as source pools accumulated on artifical substrates suspended for varying time in Douglas Lake, Michigan, and ranged from "immature" or pre-MacArthur-Wilson equilibrium (6 hr, 24 hr or 48 hr old) to "mature" or post-MacArthur-Wilson equilibrium (21, 27 or >1000 days old). The rapidity with which sources produce propagules was examined in additional experiments by removing one of an identical pair of sources after 24 hr in paired experiments. Sources used in the second experiment were immature (1 and 6 days old) and mature (17, 21 or 28 days old). In each case examined, species export was rapid and no significant differences occurred in early colonization resulting from exposure of islands to different sources. Examination of colonizing species revealed that the process was dominated by 43 species, many of which had been previously identified as pioneer species or opportunistic colonists in natural systems. Early colonists were rapidly exported by all sources regardless of source pool age.