A study of scale insects (Coccoidea) on oak trees (Quercus sp., Fagaceae) was undertaken between April and October in 2005 and 2006 in Bulgaria (B) and Greece (G), mainly in regions of beekeeping interest, to determine the sources of honeydew utilized by the bees. A total of 12 species of scale insects were collected: Coccidae: Eulecanium tiliae Linnaeus (B and G), Eulecanium ciliatum Douglas (B) and Parthenolecanium rufulum Cockerell: (B and G); Diaspididae: Targionia vitis Signoret (B and G), Lepidosaphes ulmi Linnaeus (B), Diaspidiotus wuenni Lindinger (G), Diaspidiotus zonatus Frauenfeld (G) and Diaspidiotus lenticularis Lindinger (G); Kermesidae: Kermes roboris Fourcroy (B) and Kermes gibbosus Signoret (B); Eriococcidae: Eriococcus sp. (B and G), and Asterolecaniidae: Asterodiaspis repugnans Russell (B and G). Only 6 of these species are honeydew-producing insects, and only 3 of them (Eulecanium tilliae, Parthenolecanium rufulum and Eriococcus sp.) were found in both Bulgaria and Greece. As Parthenolecanium rufulum is the most widespread species, it is most likely to be the main source of honeydew from Coccoidea. Among the species collected, the asterolecaniid Asterodiaspis repugnans (Russel) is a new record for Bulgaria, and the diaspidid Diaspidiotus wuenni (Lindinger) is a new record for Greece. Biological information, such as, host plant, developmental stage, distribution and some phenological data are given.