1. The monosynaptic projection of single Ia muscle spindle afferents to motoneurones of identified type has been examined by spike triggered averaging in barbiturate anaesthetized cats. 2. The amplitude of averaged individual monosynaptic e.p.s.p.s in medical gastrocnemius motoneurones from homonymous Ia afferents was clearly related to the motor unit mechanical properties. It was largest in type S units, intermediate in type FR and smallest in type FF. 3. The projection frequency of homonymous Ia afferents to medial gastrocnemius motoneurones followed the same order. 4. Similar trends were evident in both respects for the heteronymous projections of triceps surae. 5. The mechanical property of units which related most directly to e.p.s.p. size was muscle unit contraction strength, a single continuous relation existing throughout all three unit types. 6. On the basis of motor axon conduction velocity measurements, it appeared that motoneurone size could provide a simple explanation for e.p.s.p. size in type S and type FR units, but not in type FF. The latter had unduly small e.p.s.p.s relative to their axon conduction velocity, indicating a qualitatively different presynaptic organization for FF motoneurones from the others. 7. The data make it possible to predict that Ia afferent excitation alone would cause recruitment of units in order of increasing contraction strength and that this order would also obtain if a substantial part of the excitation came from other sources with uniform effect on all unit types. 8. It is shown that a consequence of the observed organization is that the gain of the Ia mediated stretch reflex would be approximately proportional to the developed force.