The family Anablepidae is formed by three species distributed from Central America to the northern part of South America. In Brazil only two species occur, A. anableps and A. microlepis. This paper reports on the population structure, reproduction and embryonic aspects of these two species, based on material collected from August 2001 to May 2002, at Paracauari River (00043' Se 480 31' W). Collections were made using trawls, puçá and tarrafa nets. Collected material was measured (mm), weighed (g), and the gonads and embryonic development were examined macroscopically. A total of 1,090 A. anableps and 283 A. microlepis were collected and analyzed. The two species form a single school that fragments in several groups, but they differ in mean length, A. anableps is larger (= 139 mm) than A. microlepis (= 128 mm). As for sexual parity, females are larger than males (females of A. anableps = 152 mm and males = 136 mm; females of A. microlepis = 150 mm and males = 118 rnm); in numbers of males and females A. anableps showed no significant differences (?2 = 0.051), while in A. microlepis (?2 = 5.12) females predominated. Tralhoto is viviparous and reproduces all year round. Each female generates an average of 15 oocytes, which, before being fertilized, remain loose inside the ovaries, and after fertilization adhere to their walls until hatching. When the embryo reaches 30 mm in length (stage 3), the ovaries fill with blood and abortion is frequent. Only 1 to 5 pups reach the final stage of development (45 mm), when they are delivered. There is possibly a mutual benefit for the type of aggregation between A. anableps and A. microlepis; and a trophic relationship between the maternal organism and the embryo.
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