To further analyze the interobserver agreement in the use of a new protocol for Structured Observation of Motor Performance (SOMP-I), 35 infants between one and six months of corrected age were assessed by two physiotherapists. Twenty-seven preterm and full-term infants who consecutively attended a follow-up clinic were interspersed with five neonatally healthy infants and three infants admitted for physiotherapy. The protocol was designed to assess the level of motor development and the degree of deviation from the motor performance described at the achieved level of development. The assessment of levels of development showed good interobserver agreement when measured by the percentage agreement method (≥ 80%), by kappa statistics (≥ 0.62), and as the Spearman rank-order correlation coefficient (≥ 0.80). The majority of the assessments of degree of deviation showed agreement values from fair to good. Low-interobserver-agreement values were found for the items describing the sitting position (66%) and for the trunk in the supine and prone positions (66%). Thus, good interobserver agreements were achieved for most items on the SOMP-I.