Although many children experience difficulties for reading (RLD) and arithmetic (ALD) for a variety of reasons, Developmental Dyslexia (RD) and Developmental Dyscalculia (DD) appear to be due to a brain dysfunction that causes deficit in lower-level processes. Here, we report two large scale prevalence studies based on this neurocognitive approach. In the first study (HCS), 11,652 children between 2nd and 9th grades of a Havana City municipality were evaluated. The second study (NS) included 16,097 children of 3rd and 6th grades from urban and rural schools of the country. In the first stage of testing, achievement tests were administered and the prevalence of RLD and ALD was obtained. In the second stage, theoretically-motivated neuropsychological batteries were used to assess the prevalence of RD and DD. In the HCS the prevalence of RLD and ALD was 14.8% and 8.17% whereas in the NS was 6.35% and 7.19%, respectively. The prevalence of RD and DD in the HCS was 2.57% and 5.9% whereas in the NS was 4.27% and 3.17%, respectively. For all disabilities there was a significant preponderance of boys respect to girls. Our results support that RD and DD children comprise a subset with highly selective deficits to basic-level representations. These deficits can be measured by item-timed tests of basic capacities.