A novel rifamycin derivative having the formula (I): ##STR1## wherein X.sup.1 is an alkyl group with 1 to 6 carbon atoms or a cycloalkyl group with 3 to 8 carbon atoms; X.sup.2 is a hydrogen atom or an alkyl group with 1 to 4 carbon atoms; R.sup.1 is hydrogen atom or acetyl group; A is a group represented by the formula: ##STR2## wherein R.sup.2 is an alkyl group with 1 to 4 carbon atoms or an alkoxyalkyl group with 2 to 6 carbon atoms and R.sup.3 is an alkyl group with 1 to 6 carbon atoms or an alkoxyalkyl group with 2 to 6 carbon atoms, or a group represented by the formula ##STR3## wherein ##STR4## is a 3 to 9 membered cyclic amino group with 2 to 8 carbon atoms, R.sup.4 is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, or salts thereof, a process for preparing the same and antibacterial agents containing the same as an effective ingredient.The rifamycin derivative of the present invention having the formula (I) shows a strong antibacterial activity against the Gram-positive bacteria and the acid-fast bacteria.