In this paper it is argued that an expert system requires more than factual knowledge before it can display expertise in a given domain. The additional knowledge consists of the heuristics or 'rules of thumb' used by an expert to manipulate and interpret the factual knowledge. The knowledge acquisition phase of an expert system project involves determining the factual knowledge (which may be obtained from published sources) and the heuristics used by an expert to manipulate that knowledge--these heuristics can only be obtained from an expert. In reviewing existing biological expert systems it is apparent that many contain only the factual knowledge relating to the domain, and lack the heuristics that enable such systems to show expertise. This paper reviews a number of knowledge acquisition techniques which could be used for acquiring heuristic knowledge and discusses when their use is appropriate. The knowledge acquisition techniques discussed are those suitable for the development of small-scale expert systems as these are most likely to be of interest to biologists. The techniques include the use of questionnaires, interview techniques and protocol analysis; particular emphasis is placed on a modification to the 'twenty questions' interview technique which was developed specifically to elicit taxonomic knowledge relating to water mite identification.