This study has investigated the presence of conservation of number in 117 children aged 2-5 to 4-4. Using a more complete method than seen in previous conservation studies with this age group (Braine, Bruner, Mehler & Bever), 2% of the Ss were found to be conservers. Comparisons were also made with the results of similar procedures used with Ss aged 4-3 to 6 years. Age and SES differences in conservation attainment were found but no sex differences. Further, the difference in the conservation score between the SES groups in the younger sample (2-5 to 4-4) was found to be equivalent to about one year while the gap between the social classes at ages 4-5 to 6 was nearly two years. The importance of the methodology in measuring number conservation is also discussed.