Opaque polyester film containing fine carbon black particles

A magnetic recording medium has a biaxially-oriented polyester film base which includes carbon black beads having a median diameter from 50 to 500 nm and is substantially free from beads or agglomerates more than 5 micrometers in diameter. The beads provide opacity such as is often required of the flexible recording disk of a diskette. The film base may be either a single layer, throughout which the beads are uniformly dispersed, or it may be multi-layer, e.g., a central layer containing the carbon black beads and two surface layers which are free from the beads.