Abstract: There are provided new cephem compounds (synisomers) represented by the following general formula (I): ##STR1## wherein R.sup.1 means a lower group, R.sup.2 denotes an ester-forming group of the carboxyl group, and the 4-methylthiazolyl group and the cephem moiety are cis to each other relative to the carbon-carbon double bond of the substituted vinyl group in the side chain at the 3-position of the cephem nucleus. The cephem compounds exhibit strong antibacterial activities not only against resistant strains of bacteria and gram-negative bacteria but also against gram-positive bacteria and moreover have very low toxicity.