Physiological and electron microscopic studies have shown that in different teleosts the pineal organ has a photosensory function (see Discussion). This partly contradicts earlier light microscopic results in teleosts which associate strongly folded and highly vascular pineal organs, and their acidophilic inclusions with secretory function. Although the pineal organ of the eel is “much reduplicated and highly vascular”, there is no clear evidence of secretory activity in this form (see Tilney and Warren, 1919). The pineal organ of the guppy belongs to a different structural type and consists of a stalk and an end-vesicle with a narrow lumen. Pflugfelder (1953) has concluded after epiphysectomy that the pineal organ of the guppy may have an endocrine function. In view of these inconsistencies in the interpretation of teleost pineal systems, the pineal organs of the eel (Anguilla anguilla) and the guppy (Lebistes reticulatus) were examined with the electron microscope. Although the tissue layers covering the pineal organ are very different in the two species, the pineal organs in both species have well-developed receptor cells with characteristic outer segments. In the pineal organ of the eel, synapses with synaptic rods (ribbons) are observed. The pineal organs of the eel and the guppy show all of the structural elements typical for a pineal sense organ. Evidence obtained from other teleost species (e.g., Esox lucius) shows that pineal organs with a sensory apparatus may also form biogenic amines (Owman and Rudeberg, 1970); this has not yet been investigated in the eel or the guppy. The present results indicate that caution should be exercised in classifying teleost pineal organs after conventional light microscopic examination into predominantly sensory and nonsensory (secretory) types. A parapineal organ is present in the adult eel but is missing in the guppy.