Two fundamental aspects of lamprey biology were examined to provide tools for population assessment and determination of critical habitat needs of Columbia River Basin (CRB) lampreys (the Pacific lamprey, Lampetra tridentata, and the western brook lamprey, L. richardsoni). We evaluated the usefulness of current diagnostic characteristics for identification of larval lampreys (i.e., pigment patterns) and collected material for development of meristic and morphometric descriptions of early life stage CRB lampreys, and we determined the effects of temperature on survival and development of early life stage CRB lampreys. Thirty-one larval lampreys were collected from locations throughout the CRB and transported to the Columbia River Research Laboratory. Lampreys were sampled at six-week intervals at which time they were identified to the species level based on current diagnostic characteristics. Sampling was repeated until lampreys metamorphosed, at which time species identification was validated based on dentition, or until they died, at which time they were preserved for genetic examination. These lampreys were sampled 30 times with two individuals metamorphosing, both of which were consistently identified, and subsequently validated, as Pacific lampreys. Of the remaining lampreys, only one was inconsistently identified (Pacific lamprey in 83% of the sampling events and western brook lamprey in 17% of the sampling events). These data suggest that pigmentation patterns do not change appreciably through time. In 2001 and 2002 we artificially spawned Pacific and western brook lampreys in the laboratory to provide material for meristic and morphometric descriptions. We collected, digitized, preserved, and measured the mean chorion diameter of Pacific and western brook lamprey embryos. Embryos ranged in development from 1 d post fertilization to just prior to hatch, and were incubated at 14 C. Mean chorion diameter was greater and more variable for Pacific lampreys (mean {+-} SD; 1.468 {+-} 0.107 mm, N = 320) than for western brook lampreys (1.237 {+-} 0.064 mm, N = 280). An unpaired t-test showed that the difference in mean chorion diameter between species was highly significant (t = 32.788, df = 528.62, P < 0.0001). For larvae, we collected, digitized, and preserved 156 individuals from each species. Eight homologous landmarks defining a two-cell truss network with two appended triangles were selected for morphometric analyses and species discrimination. A full model discriminant analysis correctly classified 92% of the Pacific lampreys and 93% of the western brook lampreys in a classification data set. When applied to a test data set, the classification functions correctly classified 91% of the Pacific lampreys and 85% of the western brook lampreys. A backward elimination discriminant analysis removed four variables from the full model, and the reduced model correctly classified 91% of the Pacific lampreys and 93% of the western brook lampreys in a classification data set. The reduced model classification functions correctly classified 91% of the Pacific lampreys and 85% of the western brook lampreys in a test data set. In 2001 and 2002 Pacific and western brook lampreys were artificially spawned and resulting progeny were reared in the laboratory at 10 C, 14 C, 18 C, and 22 C. The estimated temperature for zero development was 4.85 C for Pacific and 4.97 C for western brook lampreys. Survival was greatest at 18 C followed by 14 C, 10 C, and 22 C, with significant differences observed between 22 C and other temperatures. Overall survival was significantly greater for western brook than for Pacific lampreys, although the difference in proportion of individuals surviving was only 0.02. Survival to hatch was significantly greater than survival to the larval stage with a difference of only 0.03. The proportion of individuals exhibiting abnormalities at the larval stage was greatest at 22 C followed by 18 C, 10 C, and 14 C, with significant differences observed between 22 C and other temperatures.