Heme proteins are involved in a wide variety of biological reactions, including respiration, oxygen transport and oxygen metabolism [1]. The heme prosthetic group is synthesized in almost all living organisms except for a few pathogenic bacteria and trypanosomatids that use blood as food [2] [3]. There is a general belief that all nucleated animal cells synthesize heme [1] [4]. However, blood-feeding arthropods ingest enormous amounts of vertebrate blood in a single meal and the heme pathway has not been studied in these animals. We have examined heme synthesis in two hematophagous arthropods - the blood-sucking bug Rhodnius prolixus and the cattle tick Boophilus microplus. We show that R. prolixus makes heme and has a fully operative heme biosynthetic pathway, while B. microplus does not. To our knowledge, this is the first report of an animal that does not synthesize its own heme and relies solely on the recovery of heme present in the diet. Because of the inability of Boophilus to synthesize heme and its ability to deal efficiently with large amounts of free heme, we propose this organism as a good model for studying heme transport and reutilization in animal cells.