Three aspects of aphid host selection by parasitic larvae of Allothrombium pulvinum Ewing (Acari: Trombidiidae) were examined using two-choice tests in the laboratory. Seven aphid species were studied for host selection by larval mites: Cryptomyzus alboapicalis (Theobald), Liosomaphis berberidis (Kaltenbach), Macrosiphum rosae L., Myzus ornatus Laing, Myzus persicae (Sulzer), Periphyllus testudinaceus (Fernie), and Dreoanosiphum platanoidis (Schrank). In host species selection tests, larval mites preferred M. persicae to M. ornatus, which was as attractive to the mite parasites as M. rosae. Larval mites preferred M. rosae to C. alboapicalis. Aphidinae as represented by M. persicae, M. rosae or C. alboapicalis was more preferable to the mite parasites than P. testudinaceus of the Chaitophorinae, which was as attractive as D. platanoidis of the Drepanosiphinae. In host size selection tests, significantly more mites selected the large host than small host of L. berberidis and D. platanoides. The large and small hosts of P. testudinaceus, M. persicae and M. rosae were similarly attractive to larval mites. The size of the host aphid affected the choice by the mite parasite between a parasitized and an unparasitized host of M. persicae and P. testudinaceus, but not of L. berberidis nor of D. platanoidis. For P. testudinaceus and D. platanoidis, parasitized large hosts had higher probabilities of being attacked further than unparasitized large host, but the pattern was reversed for small aphids. These results were discussed in light of the previous studies and hypotheses on aphid host selection by mites.