Master Mind is a commercial game, popular in the 1970s, that has been extensively used in psychological and educational research. Here, a dynamic assessment format is developed in order to test metacognitive skills in individuals with mental retardation. The main objective is to obtain valid scores that adequately reflect individual differences among low-functioning individuals. Sixteen persons with mental retardation and 13 of average intelligence were compared on static and dynamic Master Mind scores. Scores for subjects of the mental retardation sample were also obtained on the ICAP adaptive functioning score (Bruininks et al., 1986), and on a metacognitive functioning scale, the "Cognitively Enriched Behaviour Matrix" (Mora and Mora-Merchn, 1995). The latter was administered while participating in a thinking skills programme, "Comprehending and Transforming" (Mora, 1997) following assessment with Master Mind. Differences between persons with average intelligence and mental retardation were statistically significant on all processes and both on dynamic and static scores. More importantly, dynamic scores on Master Mind were related to adaptive behaviour (d = 0.482, p < 0.001), and predicted metacognitive functioning in the learning environment of the thinking skills programme (d = 0.727, p < 0.001). Neither IQ nor static scores related significantly to these criteria.