Mechanical and electrical properties of 22 glass-and asbestos-fortified thermoplastic resin systems are presented.Large increases in tensile and flexural strength are noted with the glass- and asbestos-fortified nylon compounds; increases for other fortified resins are also qualitatively detailed. Flexural modulus data also demonstrate the ability of fortifiers to increase rigidity dramatically. Unnotched izod impact strengths are generally reduced by addition of fiber fortifiers, while notched izod impact strengths are increased for some resin systems and decreased for others. Increases in heat deflection are generally realized.The d-c electric properties of dielectric strength and arc resistance are generally enhanced in glass fiber-fortified thermoplastic resins. The a-c electric properties of dielectric constant and dissipation factor are similarly increased. Both increases and decreases in volume resistivity data are noted with the addition of fiber fortifiers. Slightly reduced volume resistivities and increased dissipation factors at 60 Hz are noted with asbestos-fortified nylon resins.The addition of flame-retardant additives to fortified thermoplastic resins results generally in reduced physical and electrical properties.