Heart extracts from over 100 species of birds were subjected to starch-gel electrophoresis at pH 7. The "supernatant" form of malate dehydrogenase, an enzyme present in every extract, was then located on the gels by a specific staining method. The mobility of this enzyme shows very little interspecific variation. Nearly all birds tested have a supernatant malate dehydrogenase that moves as fast as the chicken enzyme. Those species with an enzyme of unusual mobility are of taxonomic interest. For example, hummingbirds and swifts, which are usually considered as two suborders of Apodiformes, are unique among the birds tested in having an enzyme that moves 63 percent as fast as the chicken enzyme. This finding appears to confirm the unity of the Apodiformes, an order whose unity has long been open to question. Similarly all families tested in the shorebird order (Charadriiformes) are unique in having an enzyme that moves 55 percent as fast as the chicken enzyme. The unity of this order was also previously open to question.