Organ Pipes NP in NW Melbourne is a replanted eucalypt woodland where timber bat boxes of Stebbings' English design were installed in 1992 and first used late in1994. The bats found have been over 90% Gould's and 7% Large Forest Bats. Boxes of 6 different designs were tried over 15 years, differing in internal dimensions and entrance slit size, ranging from 0.002m³ to 0.009m³. The first ten boxes are used only in warm months, September to February. Three fallen boxes were replaced with smaller pinus boxes, which are used over more of the year. In 1996 some boxes, conforming more to Stebbings' design, were added, which are used for a yet wider seasonal range. Another set of smaller boxes have been used mainly in cooler months, and are avoided in summer. A set of boxes made of 45mm timber are also used in colder months. Annual patterns of box usage are examined for the different box designs - some show slow build-up, usage of others fluctuate up and down in cycles of several years, or are well used but avoided for some whole years, or are little used but come briefly into favour for one year. Box orientation affects usage patterns, those facing west or east being used much more than those facing north or south, and in different seasonal patterns. Where two boxes share a tree, complementary patterns may appear, one favoured in warm months, the other in colder months. An unexpected increase in usage in 2007-8 involved two bouts of breeding. Implications for study of natural hollows are discussed.