Appropriate choice of biomaterial supports is critical for the study of capillary morphogenesis in vitro as well as to support vascularization of engineered tissues in vivo. Self-assembling peptides are a class of synthetic, ionic, oligopeptides that spontaneously assemble into gels with an ECM-like microarchitecture when exposed to salt. In this paper, the ability of four different self-assembling peptide gels to promote endothelial cell adhesion and capillary morphogenesis is explored. Human umbilical vein endothelial cells (HUVECs) were cultured within ionic self-assembling peptide family members, RAD16-I ((RADA)4), RAD16-II ((RARADADA)2), KFE-8 ((FKFE)2), or KLD-12 ((KLDL)3). HUVECs suspended in RAD16-I or RAD16-II gels elongated and formed interconnected capillary-like networks resembling in vivo capillaries, while they remained round and formed clusters within KFE-8 or KLD-12 gels. As HUVECs attach to RAD16-I and RAD16-II significantly better than the other peptides, these differences appear to be explained by differences in cell adhesion. Although adhesion likely occurs via bound adhesion proteins, there appears to be no difference in protein binding to the peptides investigated. Results indicate that, although these oligopeptides have similar mechanisms of self-assembly, their primary sequence can greatly affect cell adhesion. Additionally, a subset of these biomimetic ECM-like materials support capillary morphogenesis and thus may be useful for supporting vascularization.