Three new alkanoyl imidazoles, designated bulbimidazoles A-C (<b>1</b>-<b>3</b>), were found from the culture extract of the gammaproteobacterium <i>Microbulbifer</i> sp. DC3-6 isolated from a stony coral of the genus <i>Tubastraea</i>. The absolute configuration of the <i>anteiso</i>-methyl substitution in <b>1</b> was established to be a mixture of (<i>R</i>)- and (<i>S</i>)-configurations in a ratio of 9:91 by applying the Ohrui-Akasaka method. Compounds <b>1</b>-<b>3</b> displayed unique broad-spectrum antimicrobial activity against Gram-positive and -negative bacteria and fungi with MICs ranging from 0.78 to 12.5 μg/mL. They also exhibited cytotoxicity toward P388 murine leukemia cells with IC<sub>50</sub> in the micromolar range.