Adult: Species description based on Trueb (1986) and Duellman (2001). A medium-sized treefrog with large eyes and a very short, blunt snout (males to 34 mm, females to 41 mm). Isthmohyla lancasteri exhibits a rather amazing degree of morphological and behavioral variation depending on altitude (discussed in Trueb 1986). Dorsal: The dorsal coloration consists of mottled shades of brown, green, and grey. Ventral: The venter is greyish-white, with or without dark mottling, depending on locality. Concealed surfaces: The inner and outer surfaces of the thighs have one of two patterns: yellow with black barring or white with black spots, depending on locality. Eye: The eye is reddish-brown.
Breeding season: Breeding occurs during the dry season at high elevations, but is delayed until the onset of the rainy season at lower elevations (Trueb 1968). Breeding takes place at streams (Trueb 1986, Duellman 2001). Egg: Eggs are laid in clutches of around 20 eggs and generally placed on vegetation overhanging water (Trueb 1968). Tadpole: Tadpoles grow quite large (Duellman 2001). The bodies are oval, with a long, muscular tail and short tail fins (Duellman 2001). Tadpoles are brown, with some darker or green markings (Duellman 2001). The ventral surface is grey, or bluish white (Duellman 2001). The mouth is rather small (Duellman 2001). For further descriptive information, see Starrett (1960) and Duellman (2001). Tadpoles may be encountered in quiet pools (Trueb 1986).
Habitat: Premontane forest from 368 to 1200 m. Call: Two short, high notes (Duellman 2001). Type locality: high forest at Peralta, Costa Rica, altitude 500 meters