Two new isoflavanones, 5,3'-dihydroxy-4'-methoxy-5'-(3-methyl-1,3-butadienyl)-2'',2''-dimethylpyrano[5,6:6,7]isoflavanone (1) and 5,3'-dihydroxy-5'-(3-hydroxy-3-methyl-1-butenyl)-4'-methoxy-2'',2''-dimethylpyrano[5,6:6,7]isoflavanone (2), together with two known isoflavonoids, cristacarpin, and euchrenone b10, were isolated from the stems of Erythrina costaricensis. Their structures were established on the basis of spectroscopic evidence. These new compounds are rare isoflavanones, possessing both a 2,2-dimethylpyran substituent and a prenyl analog. The antibacterial activities of 1 and 2 against the methicillin-resistant Staphylococcus aureus were examined.