The muscular control of eyecup movements in the crab Carcinus maenas has been studied by both extracellular and intracellular recording from the nine eyecup muscles. Each muscle involved in optokinetic movements is supplied by a fast and slow axon and each consists of a histologically mixed spectrum of fibres ranging from (Felderstruktur). Muscle 19a, which only participates in the withdrawal reflex consists of phasic fibres only. In general, a fast axon preferentially innervates the phasic fibres and a slow axon the tonic ones. During optokinetic movements the muscles are activated by a complex motor output programme, which is different, not merely the reverse for movements in opposite directions. Both tonic and phasic muscle fibres are active but the latter are only active at greater amplitudes of stimulus movement. Tonic activity is responsible from maintaining the eyecup position in space and for low velocity, small amplitude movements. Phasic activity is recruited during large amplitude movements and is also responsible for fast movement and eyecup tremor. The protective withdrawal reflex overrides any other eyecup movement and involves the firing of two axons in the optic tract, one supplying a group of two, the other a group of three muscles. One of the muscles involved in this eyecup withdrawal movement away from the mid-line is also active during horizontal optokinetic movements of the eyecup towards the mid-line. It is suggested that interpretation of eyecup muscle activity is more intelligible if the whole group of muscles, rather than the individual muscles themselves, is regarded as the functional unit.