Abstract: An improved spark plug, capable of suppressing radio-frequency electromagnetic interference from internal combustion engine ignition systems such as those commonly used in motor vehicles, is disclosed. The plug comprises a shell releasably engageable with an engine, an insulator mounted in the shell, a center electrode seated within the insulator and a ground electrode structurally integral with the shell and in spark gap relationship with the center electrode. The spark plug is further characterized in that at least a portion of its insulator has a dielectric constant sufficiently high that the effective dielectric constant of the insulator is at least 30, and the relationships among the elements of the spark plug and the effective value of the dielectric constant of the insulator are such that the spark plug has a capacitance of at least 20 picofarads. Preferably, the spark plug has a capacitance of from 20 to 100 picofarads, most desirably from 30 to 80 picofarads.