Several five-membered heterocycles having a phenylazo substituent, 3-phenylazotetronic acids (2a-n), 3-phenylazotetramic acids (4a-i), 4-phenylazo-5-isoxazolinones (6a-g, 8a-f) and 5-phenylazo-4-thiazolidinones (10a-f, 12a-e), were synthesized and tested for antimicrobial activities. Tetronic acid and tetramic acid derivatives (2 and 4) inhibited the growth of gram-positive bacteria. 5-Isoxazolinone and 4-thiazolidinone derivatives (8 and 10) showed inhibitory activities against fungi as well as bacteria.