Abstract: An improved mainshaft gear retainer assembly (66, 68) for a twin countershaft (18, 18A) transmission (10) having at least one pair of floating mainshaft gears (44-46, 48-50) positioned close together in groups of two and clutchable to a mainshaft (40) one at a time by positive clutches (54, 56, 58) is provided. The improved mainshaft gear retainer assembly prevents axial movement of the two closely positioned gears and reacts the axial forces on a clutched one of the mainshaft gears toward and away from the other mainshaft gear to the mainshaft by means of thrust surfaces (78, 124, 126, 128, 130, 132, 134, 136) which are all rotating, or tending to rotate, at the same speed as the clutched mainshaft gear and the mainshaft. The gear retainer comprises relatively simple components and allows easy assembly and disassembly of the mainshaft gears and gear retainers from the mainshaft.