The present invention relates to gas-filled microspheres useful as ultrasound contrast agents. More particularly, the present invention relates to gas-filled microspheres prepared from amphiphilic poly amino acid block co-polymers and a pharmacologically acceptable water-insoluble gas. The hydrophobic block of the co-polymer is formed from amino acids possessing hydrophobic side-chains, for example, leucine, isoleucine, valine, and phenylalanine, and the hydrophilic block of the co-polymer is formed from amino acids possessing hydrophilic side-chains, for example, glutamic acid (and its ionized form, glutamate), aspartic acid (and its ionized form, aspartate), and lysine.