Purpose of this study was the problem-solving effectiveness and time required for solution under cooperative, competitive, and individual conditions of 108 sixth grade Greek children. Thirty six three-person groups, half all-male and half all-female, were given Mastermind and Questions problems across all three conditions. The sex (2) × conditions (3) A NOVA with repeated measures across conditions resulted in the most effective problem solver working individually was more effective than the cooperative condition or the competitive condition. The least effective problem solver working individually was less effective than the cooperative condition or the competitive condition. The average individual did not differ in problem-solving from the cooperative condition, as would be predicted by Johnson et al. (1981). Cooperative group interaction was more effective than competitive, but only with Mastermind. Individuals were not necessarily faster than groups. No sex differences were found. In comparing problem-solving effectiveness of individuals and groups, qualifications should be made regarding comparisons with the average individual or high individual.