Nine small-for-gestational age (SGA) and 10 appropriate-for-gestational age (AGA) fullterm infants were assessed using the Bayley MDI scale at 10 months, and infant-mother interactions observed at 6 months. Relationships among interaction parameters and development were compared with those of earlier interactions and development. At 6 months SGA and AGA mothers behaved similarly in interaction; SGA infants were more passive than AGA infants and demonstrated higher levels of state change. AGA infant vocalising frequencies at 2, 3 and 6 months were negatively related to MDI scores whereas maternal stimulation was positively related. Aspects of early SGA infant activity were negatively related to 10 month development; at 6 months several variables expressing dyadic interaction were negatively related. Ten month MDI scores were lower for the SGA than the AGA group. Socioeconomic status was related to MDI scores for AGA but not SGA infants. The results suggest that AGA and SGA infants behaved differently in interaction and that differential interaction patterns were linked with optimal development. Maternal stimulation and co-ordinated infant signalling in AGA dyads, and infant quietness and maternal activity encouraging quietness in SGA dyads, were patterns in high-scoring subjects. The exploratory nature of the study is noted.