The adap1 (ADP-ribosylation factor GTPase-activating protein [ArfGAP] with dual pleckstrin homology [PH] domains 1) gene is predominantly expressed in the mouse brain and is important in neural differentiation and development. However, the functions of adap1 in morphogenesis, locomotor activity, and behaviors in vertebrates are not fully understood. Whole-mount in situ hybridization (WISH) analysis revealed that adap1 was widely expressed in the zebrafish brain, including the forebrain, midbrain, and hindbrain, during early embryogenesis. To investigate the physiological function of the adap1 gene, we generated zebrafish adap1 mutants harboring frameshift mutations around codon 120 of adap1. The adap1 mutants containing homozygous mutant alleles exhibited no apparent morphological abnormalities at 1 day postfertilization (dpf), and the spontaneous coiling and touch response of the adap1 mutants were comparable to those of the wild-type fish. In addition, the expression of neural genes, such as emx1, mbx, and huC, was comparable between the wild-type fish and the adap1 mutants at 1 dpf. The adap1 mutants grew to adulthood without exhibiting any apparent swimming defects. The adult adap1 mutants spent more time than the wild type in the center region of the open field test. In the social behavior test, zebrafish containing the adap1 mutant alleles spent more time than the wild type in the regions near the chambers where novel conspecifics swam. These results imply the involvement of the adap1 gene in regulating approach behavior to visual cues from conspecifics.