A magnetic tape cassette is provided with a pivotal closure lid pivotably secured to both sides of a casing for movement between a closed position, in which the front opening of the casing, through which a magnetic tape is accessible for loading onto a rotary head, is covered, and an open position wherein the closure lid is pivoted away from said opening to expose the tape while the cassette is in use. The magnetic tape cassette is also provided with a sliding closure member slidably mounted on the bottom of the casing and cooperative with the aforementioned pivotal closure lid. The sliding closure member is in a closed position while the pivotal closure lid is in its closed position, in which it covers a bottom clearance through which a device constituting part of a tape holding system is inserted, and in an open position while the pivotal closure lid is in its open position to allow the aforementioned device of the tape handling system to reach into the casing through the bottom clearance. A locking pin holds the sliding closure in the current one of its two positions. The sliding closure holds the pivotal closed while in its own closed position.