Abstract: A penicillin of the formula: ##SPC1##Wherein A is a substituted or unsubstituted condensed aromatic carbocyclic or heterocyclic ring, R is hydrogen or lower alkyl, X is oxygen or sulfur, Y is hydrogen, lower alkyl, lower alkanoyl or lower alkoxycarbonyl and Z is phenyl or thienyl which can be produced by reacting 6-aminopenicillanic acid or its ester with a carboxylic acid of the formula: ##SPC2##Wherein A, R, X, Y and Z are each as defined above or its reactive derivative. The said penicillin and its non-toxic salts have a broad antimicrobial spectrum against various gram-positive and gram-negative bacteria, and they exhibit characteristically a strong antimicrobial activity against Pseudomonas.