Intensive short-term (immersion) courses have been used in business and industrial settings for many years. This article documents a twelve-day undergraduate immersion course in introductory computer science. Four days per week for three weeks are used for an entire course, including programming, class discussions, homework, and hour exams; extensive experience is provided in programming microcomputers and time-shared computers with BASIC and WATIV. The article includes a description of the experience for the years 1979–1981, including the computer environment, curricular structure, and class evaluations. Comparisons are made to semester and summer session classes, and a statistical analysis is presented. It was found that as much is learned and experienced in the twelve-day course as in a semester or summer course. Evidence is provided that supports the claim that networks of microcomputers can offer more flexibility and throughput than large time-shared systems.