Colostethus trilineatus (Boulenger, 1883) is widespread in the lowlands of Amazonian Peru and Ecuador. It is characterized, in part, by small size, gray throat and Finger III strongly swollen in adult males, and an advertisement call of 10 notes with emphasized frequencies at 4920–6040 Hz. Material examined revealed extensive variation in dorsal, ventral, and thigh coloration. In numerous adult males from several localities Finger II is also swollen. The swelling of Finger II varies intraspecifically, possibly as a function of male reproductive activity.

Vocalizations of Colostethus trilineatus recorded at the type locality are temporally distinct from those reported from other localities. At the type locality, advertisement calls are composed of single notes (i.e., not couplets) of (usually) > 40 msec duration and < 90 msec between notes, whereas other calls consist of notes of < 30 msec duration produced as couplets repeated at roughly 200 msec intervals. Although these differences in amplitude modulation could indicate the existence of two species, the calls at the three localities are spectrally identical (all have emphasized frequencies primarily between 5 and 6 kHz) and the specimens examined from these localities are morphologically indistinguishable. For the time being, we interpret the differences in temporal patterning as intraspecific variation.

Colostethus melanolaemus is a new species known from the type locality near the mouth of the Napo River and the upper Ampiyacu River. Colostethus melanolaemus most closely resembles C. trilineatus and C. juanii Morales, 1994, but it differs from them in male throat color, swelling of Finger III, snout-vent length, and advertisement call. Although this species reaches a snout-vent length of nearly 24 mm, Finger IV is shorter than or equal to Finger II, and the distal subarticular tubercle is absent. The advertisement call of C. melanolaemus usually consists of four notes with emphasized frequencies of 3840–4560 Hz. For comparative purposes, the advertisement call of C. juanii is also described.

The oblique lateral stripe, found in many dendrobatids, occurs as a diffuse, inconspicuous, pale region or group of small spots that extends from the groin to midway along the flank in Colostethus brunneus (Cope, 1887), C. juanii, C. melanolaemus, C. trilineatus, and several other similar species of Colostethus. This pattern differs from the states found in most species of Colostethus.

Colostethus alessandroi is a new species found between about 800–1500 m on the eastern slopes of the Andes of southern Peru. This species is most similar to C. mcdiarmidi Reynolds and Foster, 1992, but differs in amount of toe webbing, ventral coloration, snout-vent length, dorsal coloration, shank length, and tympanum diameter. Both C. alessandroi and C. mcdiarmidi have swollen third fingers in adult males. The call of C. alessandroi is composed of couplets repeated rhythmically in