This study investigates prospective elementary school teachers’ proficiencies at problem posing. The research employed qualitative case study method in order to examine the case in its natural setting and from the participants’ perspectives. The research was carried out with 162 prospective elementary mathematics teachers. A written exam, which included 10 open-ended tasks, was administered to the participants; and then semi-structured interview was conducted with 8 participants. Data were analyzed through content and discourse analysis methods. The results indicated that most of the participants were not proficient enough at posing conceptually rich and cognitively challenging problems. They displayed relatively better success at generating problems through re-formulation tasks; yet, they did this by changing story of the original tasks or the numerical values in it. Their success declined gradually when they were generating problems from semi-structured and free problem posing situations. The problems that they constructed excluded mathematical reasoning and creativity; these tasks could be resolved by the application of rules, procedures and factual knowledge. The participants’ lack of proficiency at problem posing seems to be resulting from their educational backgrounds. Thus, it is suggested that engaging prospective teachers in problem posing activities, especially in semi-structured and free situations, during their undergraduate education will contribute to the solution of this problem.