Traditional field delineation methods in bibliometric studies appear to have reached their limits when dealing with highly interdisciplinary fields such as nanotechnology or stem cell research which have recently become a focus of science and technology policy research. Researchers therefore have developed sophisticated algorithmic procedures to overcome these difficulties, hoping to collect a set of articles in a research field being studied that is complete as well as clean. The present case study explores the effect of field delineation on author co-citation analysis (ACA) studies of the intellectual structure of the library and information science (LIS) field using two different but overlapping journal sets to define the LIS field. We find that the major overall structure remains largely the same between these two views of the LIS field, which suggests that field delineation is not crucial to ACA studies of research fields, provided the emphasis of a study is on the major overall structure of a research field. The two views do however differ at a more detailed level of analysis, which suggests that studies that aim to shed light on particularly subtle research policy issues may need to pay serious attention to the way that they delineate their fields.