It has become commonplace in papers on plant competition to find statements such as outcome of replacement series depends on (e.g. Inouye and Schaffer 1981, Firbank and Watkinson 1990, Silander and Pacala 1990, Silvertown and Dale 1991). The implications of this are that we may well draw the wrong conclusions from such an experiment if we examine only a single total density, and that conclusions from replacement series (RS) should be doubted (Silvertown 1987). The accepted procedure now seems to be that RS should be repeated at a number of total densities and the results analysed by fitting response surfaces (despite the numerous pitfalls see Cousens 1991). Criticisms of RS have become common and seem to be becoming increasingly dogmatic. We now seem to have to apologise for using RS, or go to great lengths to justify our choice of this design (e.g. Akey et al. 1991, Cousens et al. 1991). Are there papers which have been rejected simply because they used RS? It seems to have gone largely unnoticed that some of those who have criticised the use of RS, or whose data have been used to criticise the design, recognise that it can be useful for some objectives (Taylor and Aarssen 1989, Firbank and Watkinson 1990). Indeed, Taylor and Aarssen (1989) state that Although (RS) designs have come under considerable criticism they may only be subject to the same logistic difficulties that face other approaches with the same objectives. Firbank and Watkinson (1990) state that the RS is ... extremely valuable for comparing the outcome of competition between two plant species under different conditions. If this is the case, it is a great shame if those unfamiliar with the data are put off using the RS design simply because of recent adverse publicity by respected commentators. Before rejecting the replacement series design outright because of its supposed density dependence, we should ask, on the basis of current knowledge of competition, (a) what would we expect to happen in a replacement series as we change density, (b) whether existing data support our predictions, and (c) whether the evidence is sufficient to reject the RS design on these grounds. Predictions from response surfaces