Extra work on unsolved problems may lead to more improvement if the new work is delayed rather than undertaken immediately after initial solution attempts. Such a result constitutes incubation in problem solving. "Unconscious work" on a problem, commonly assumed to be responsible for incubation effects, may not be necessary to observe the phenomenon. We hypothesize that fixation, a block to successful problem solving, may develop during initial solution attempts and persist, interfering with immediate extra work more than with delayed extra work. Five experiments are reported in which fixation was induced to prevent optimal performance on the initial test of Remote Associates Test (RAT) problems (e.g., Mednick, 1962). After the fixation manipulation in three of the experiments, the effects of incubation intervals were examined by retesting the fixated problems. Both fixation (poorer initial problem-solving performance) and incubation (more improvement after a delayed retest than an immediate retest) were found in all the experiments which tested for the effects. In Experiments 1, 2, and 3, misleading distractors were presented alongside the RAT problems during the initial test of the problems to cause fixation. In Experiment 4, a block of paired associates--pairing the RAT words with the misleading distractors prior to problem solving--successfully induced fixation, indicating that the distractors affected memory retrieval. In Experiment 5, a trial-by-trial technique allowed fixation and incubation to be induced and tested separately for each item. All of our findings of incubation effects appear to have depended upon the initial induction of fixation. Although the experiments may not be representative of all naturally occurring cases of incubation, they provide a methodology for the study of fixation and incubation effects in problem solving in the laboratory.