1. AQUATIC.--This habitat includes permanent streams and rivers (Pl. 10, fig. 1), some of which are clear and others muddy. In the rainy season temporary ponds form in depressions on the forest floor (Pl. 10, fig. 2); these are important as breeding sites for many species of amphibians. Aquatic members of the herpetofauna are here considered to be those species that either spend the greatest part of their lives in the water or usually retreat to water for shelter. Seven species of turtles and one crocodilian are aquatic. Of these, _Dermatemys mawi_, _Staurotypus triporcatus_, and _Pseudemys scripta ornata_ inhabit clear water, whereas _Chelydra rossignoni_, _Claudius angustatus_, _Kinosternon acutum_, and _K. leucostomum_ inhabit muddy water. _Crocodylus moreleti_ apparently inhabits both clear and muddy water, for in the dry season it lives along the clear rivers, but in the rainy season inhabits flooded areas in the forest as well. 2. AQUATIC MARGIN.--Extensive marshes were lacking in the part of southern El Petén that I visited; consequently, the aquatic margin habitat is there limited to the edges of rivers and borders of temporary ponds. _Bufo marinus_, _Rana palmipes_, and _Rana pipiens_ are characteristic inhabitants of the aquatic margin, although in the rainy reason _Bufo marinus_ often is found away from water. Observations indicate that _Tretanorhinus nigroluteus lateralis_ inhabits the margins of ponds and streams and actually spends considerable time in the water. Although _Iguana iguana rhinolopha_ is arboreal, it lives in trees along rivers, into which it plunges upon being disturbed. Species included in this category are those that customarily spend most of their lives at the edge of permanent water. Frogs and toads that migrate to the water for breeding and the snakes that prey on the frogs at that time are not assigned to the aquatic-margin habitat. 3. FOSSORIAL.--Characteristic inhabitants of the mulch on the forest floor are _Bolitoglossa moreleti mulleri_, _Lepidophyma flavimaculatum flavimaculatum_, _Scincella cherriei cherriei_, _Ninia sebae sebae_, _Pliocercus euryzonus aequalis_, and _Micrurus affinis apiatus_. Other species of snakes that spend most of their lives above ground often forage in the mulch layer; among these are _Coniophanes bipunctatus biserialis_, _Coniophanes fissidens fissidens_, _Coniophanes imperialis clavatus_, _Lampropeltis doliata polyzona_, and _Stenorrhina degenhardti_. Among the amphibians, at least _Hypopachus cuneus nigroreticulatus_, _Eleutherodactylus rostralis_, and _Syrrhophus leprus_ are known to seek shelter in the mulch.