The microbial loop as a leading concept in marine microbiology gained wide recognition in the 1980s, but it has roots extending back to the 1930s when microbiologists first began to take a more dynamic approach to investigating the roles of bacteria in ocean food webs and biogeochemical cycles. Here we present a history of the microbial loop concept with emphasis on the period starting in 1930, when marine bacteriologists in Russia and the West began to study explicitly the roles of marine bacteria in the sea. Selman Waksman at Woods Hole and Claude ZoBell at La Jolla relied on colony counts on agar plates as the basis of their work. We suggest that failure to accept direct microscopic evidence of high numbers of bacteria in seawater retarded conceptual development in the West well into the 1970s. Easterners pioneered direct count and radioisotopic techniques and created a dynamic marine microbiology integrating bacteria as important components of marine food webs by the 1960s. Yurii Sorokin and colleagues carried out extensive experimental studies of bacteria as food for marine grazers and provided data for Mikhail Vinogradov and his group to write the first numerical simulation models of ocean ecosystems incorporating microbial components. It had little impact on the Western modeling community, as other Russian work of the times. In spite of continuing technical shortcomings in the field, Lawrence Pomeroy constructed a new conceptual model, providing a synthesis pointing the way toward a modern view of marine microbial ecology that finally matured technically and conceptually in the West in the early 1980s.