PLATES IX-XI THE use of firm agar (FA) to inhibit the swarming of Proteus and Clostridium spp. was first reported by Hayward and Miles (1943). A description of its preparation and use followed (Hayward, 1945(Hayward, , 1947. Some medical bacteriologists have favoured FA to prevent the swarming of Proteus spp. (Stokes, 1960;Crowley, Bradley and Darrell, 1969), but more recently its value has been questioned because it sometimes failed to prevent swarming (Stokes, 1975) and reduced the growth of certain clostridia (Williams, 1971 ;Willis and Williams, 1972).The ideal anti-swarming medium should be suitable for all swarming bacteria, non-inhibitory to the growth of any bacterium to be isolated from mixtures that include swarmers, and easy to prepare. No medium satisfies all of these criteria. The chief disadvantages of FA are the experience and special attention required for its preparation, and the unfamilar appearance it gives to colonies of common bacteria (Crowley et al., 1969;Smith, 1972).The present work was undertaken to provide a detailed description of the preparation of FA media, an assessment of the effectiveness of FA in preventing the swarming of proteus colonies, information on the effect of FA on surface viable counts of clinically important bacteria, and an account of the colonial morphology on FA of a variety of bacteria likely to be isolated in clinical laboratories. MATERIALS AND METHODSOrganisms Proteus mirabilis and Proteus vulgaris cultures (167 and 14 respectively), all of which swarmed on " routine " nutrient agar (see below), were used to test the inhibition of swarming by FA. Of the P. mirabilis cultures, 164 were freshly isolated; they came from faeces (87), urine (56), respiratory tract (7), wounds (6), genital tract (4), ear (l), eye (l), water (1) and sewage (1). The remaining three were from culture collections, namely, the National Collection of Type Cultures, London (strain NCTC9559), the Monash University Culture Collectioii and the Queensland University Culture Collection. Of the P. vuZgaris cultures, nine were freshly isolated; they came from faeces (6), urine (2) and the nose (1). The remaining five were from culture collections, namely, the National Collection of Type Cultures, London (strain NCTC4636), the Monash University Culture Collection, the The organisms used for experiments on viable counts and colonial morphology were from culture collections, except for the culture of Neisseria gonorrhoeae which was freshly isolated.The identities of all cultures were confirmed by the methods of Cowan and Steel (1974). MediaThe term " routine " is used to describe media containing 1.2% agar. The preparation of FA media required attention to detail. Care was needed to ensure that the period of steaming was sufficient to dissolve the agar completely. Boiling-over during autoclaving was prevented by the use of flasks that were large in proportion to the amount of medium, e.g., a 500-ml flask was used for 100 ml of medium and a 750-ml flask for 200 ml of medium; it was also necessary to...