In the summer of 1940 an outbreak of paratyphoid fever occurred in Kettering and in the districts surrounding it. The total number of cases was 222. Most of them were mild, and the period of vague symptoms before a doctor was called in was in some as long as three weeks to a month. A few cases were more severe, and one or two at the beginning showed toxic symptoms which led to a diagnosis of meningitis. There was one death. The centre of the epidemic was in Kettering, but the surrounding districts were also affected. The population of this area in peace time is about 133,000: it has been somewhat increased by the influx of evacuees. The age and sex distributions are shown in Table 1.The incidence was greater in women and children than in adult males. Fig. 1 shows in graphic form the number of notifications week by week. In a few instances the agglutination test of the patient's serum was the only examination made but in the majority Bact, paratyphosum B was isolated either from the blood, faeces or urine. The methods used in isolating the infecting organisms will be described in a separate paper. Many of the cases which were excreting small numbers of paratyphoid bacilli for only a short period were detected only by the use of selective ‘enrichment.’ media, and the experience gained in the epidemic showed that a negative result based only on the use of a comparatively non-selective medium such as McConkey agar is of little value and may be dangerously misleading. Most of the specimens were examined at Leicester Emergency Public Health Laboratory, but a few, especially at the beginning, were sent to private laboratories.