This study tests the effect of long-term artificial development of a robot on users' feelings of social presence and social responses toward the robot. The study is a 2 (developmental capability: developmental versus fully matured) x 2 (number of participants: individual versus group) between-subjects experiment (N = 40) in which participants interact with Sony's robot dog, AIBO, for a month. The results showed that the developmental capability factor had significant positive impacts on (a) perceptions of AIBO as a lifelike creature, (b) feelings of social presence, and (c) social responses toward AIBO. The number of participants factor, however, affected only the parasocial relationship and the buying intention variables. No interaction between the two factors was found. The results of a series of path analyses showed that feelings of social presence mediated participants' social responses toward AIBO. We discuss implications of the current study on human-robot interaction, the computers are social actors (CASA) paradigm, and the study of (tele)presence.