In 1972, two children representing a case of severe developmental retardation were discovered and taken into custody. The children, an older sister (referred to as F) and a younger brother (referred to as G), were found to have achieved no more than the physical and mental age level normal to children 1 year of age or less, in spite of their actual respective chronological ages of 6 and 5. Investigation revealed that the children's developmental retardation was due both to extreme social isolation and to complex deprivation. By following their sensorimotor, linguistic, cognitive, and socio-emotional development from the time of discovery to the date of this writing, we have found that their physical and motor development or recovery has proceeded smoothly, whereas their linguistic and cognitive development has continued to show such weaknesses as defective functioning of internal speech (Vygotsky, 1962) and poor ability to deal with abstract, linguistic subjects. F and G have continued to exhibit a tendency toward undersocialization, but this has at times been rather a positive factor in the process of attaining ego identity during adolescence, especially for G, serving to protect him from unreasonable social pressures to conform to group behavior. In combination with other cases of severe deprivation, the case of F and G holds some interesting implications for theories of human development, particularly the notion of critical periods.