Many psychological theories point to the importance of siblings in individual personality development. The impact of sibling status on interpersonal and achievement orientation was examined with undergraduates (N=1782) who completed a series of objective personality measures and a background questionnaire. Sibling status was defined in terms of four groups: only, first- middle-, and last-borns. First-borns scored significantly higher in self-esteem than last-borns; only and middle-borns did not score differently from any of the other groups. Only children exhibited a stronger internal locus of control and scored higher on the self-centeredness item than other groups. No relationship was found between sibling status and loneliness. In terms of achievement, first- and middle-borns scored more competitively than last-borns. Only and first-borns had higher aspirations than later-borns. No sibling status effects were found in the areas of mastery, willingness to work, or personal concern about the costs of achievement. Results suggest that the impact of gender and family size on these sibling status effects are areas in need of further assessment. (NRB)