Background: The Foundation Phase in Wales is a play-based curriculum for pupils aged 3-7 years old. Children learn through more holistic areas of learning in place of traditional subjects. As such, the subject of physical education in its traditional form no longer exists for pupils under the age of 7 in Wales. In light of the role of physical education in developing physical literacy and in particular the importance of this age group for laying the foundations of movement for lifelong engagement in physical activity, the disappearance of physical education from the curriculum could be deemed to be a concern. Purpose: The purpose of this study was to explore the Foundation Phase as a naturalistic intervention and examine its contribution to the development of physical literacy. Participants and setting: Participants included year 1 pupils (N = 49) aged 5 and 6 from two schools in contrasting locations. A smaller group within each class was selected through purposive sampling for the repeated measures assessments (N = 18). Research design and methods: A complementarity mixed-method design combined quantitative and qualitative methods to study the Foundation Phase as a naturalistic intervention. Quantitative data were generated with the Test of Gross Motor Development-2 administered to the sample group of children from both schools as a quasi-repeated measure, the physical competence subscale of the Pictorial Scale of Perceived Competence and Social Acceptance and the Leuven Involvement Scale for Young Children. Qualitative data were generated throughout the study from the analysis of video and field notes through participant observation. Data from the mixed methods were analysed through complementarity to give a rich insight into pupils' progress and experiences in relation to physical literacy. Results: Overall analysis of the data from TGMD-2 showed significant improvements in the Gross Motor Quotient and Locomotor skills from T1 to T3, but no significant improvement in object control. Data from qualitative methods were analysed to explore processes that may account for these findings. Video and field notes complement the quantitative data highlighting that children were developing their locomotor skills in many aspects of their learning. Observations using the Leuven Involvement Scale indicated that children had high levels of involvement in their learning and apparent in video and field notes was pupils' motivation for movement. Paired sample "t"-tests (N = 18) conducted on the Harter and Pike perceived physical competence six-item score subscales (T1 and T3) indicated a significant difference in the mean perceived physical competence scores on the six-item scale between T1and T3. Qualitative data explored pupils' confidence for movement in many areas of learning. Conclusion: The combination of quantitative and qualitative data indicates that the Foundation Phase is an early childhood curriculum that lays the foundations of physical literacy with the exception of aspects of the physical competence, specifically object control skills. Although these skills only contribute to psychomotor aspects of physical literacy they are strongly associated with later engagement in physical activity. The development of specific physical skills such as object control skills may need more specialist input with early childhood pedagogy teachers trained in motor development to see significant improvements.