The marginal fibers (mf) constitute a major fiber component of the teleostean optic tectum, and this report deals with the physiological properties of these fibers and their postsynaptic elements. The mf are unmyelinated axons which originate at the torus longitudinalis and run lateralwards, parallel to one another, along the tectum's most superficial layer (stratum marginale). Here they synapse upon the dendritic arborizations of the pyramidal (p) neurons. These arborizations originate from a single apical dendritic shaft which, near the soma, receives retinofugal axon terminals. The p-neurons also have a basal dendritic shaft and a descending axon, both of which branch out horizontally at the stratum griseum centrale (SGC). The mf were stimulated through surface microelectrodes, and field potentials were recorded on-beam throughout the tectal thickness by means of micropipettes. The mf action potential (M-spike) may show two negative subpeaks which propagate at 0.20 and 0.16 m/s. Its refractory period is followed by a period of increased amplitude and decreased latency. The M-spike is followed by a series of slow waves, namely: (a) The S-wave, which probably represents the monosynaptic depolarization of the p-neuron's apical arbor; (b) The N-wave which possibly represents an active current sink at the point where these dendritic arborizations merge into the apical dendritic shaft; (c) The D-wave, recorded at the SGC, which possibly represents the activation of the p-neuron's axon and terminals (and perhpas also basal dendritic shaft and branches); and (d) the L-wave, which might represent a later depolarization of the p-neuron's apical arborizations. Morphologically and electrophysiologically, there are several similarities between the tectal mf/p-neuron system and the cerebellar parallel fiber/Purkinje cell system.