Our previous studies had shown that cytosolic 5'nucleotidase-I (cN-I) is expressed in several tissues in pigeons, including brain and several different skeletal muscles. We observed that cN-I mRNA levels varied among different pigeon muscles. Initial quantification of the differences revealed that about 5-10 times more of cN-I transcript was present in red, oxidative muscles (breast muscle and gastrocnemius) than in white ones, composed of glycolytic fibers (biceps brachii). We had found this observation very intriguing and decided to compare human skeletal muscles distribution of cN-I with the type of oxygen metabolism. Our screen involved 60 samples of several human muscles and we assayed the correlation between the amount of transcripts of cN-I and myoglobine, which we took as a measure of oxidative-slow twitch fibers. Our question was whether in humans, cN-I presence in skeletal muscles was related to their fiber composition. If that was the case, then cN-I expression could serve as a tool to assess the percentage of oxidative fibers in any given human muscle sample, where myoglobine expression could not be readily measured. After quantification of expression of both genes, we concluded that there was no correlation between expression of cN-I and fiber type. Therefore, contrary to the pigeon muscles, cN-I did not reflect the ratio of oxidative fibers to the total mass of the muscle sample in humans. That difference indicated that there were certain mechanisms that differentially regulated the expression of cN-I in muscle tissues of mammals and lower vertebrates.