1. The reactions, under field conditions, of Lucilia sericata (Mg.) to various substances placed on sheep are described.

2. Attempts to produce either attraction or oviposition by placing cystine or cysteine hydrochloride in the living fleece gave negative results.

3. Several organic sulphur compounds (see Table 1) of a type which might arise from the breakdown of cystine have been tested. All showed some power of attraction for L. sericata. Ethyl mercaptan and dimethyl disulphide were the most powerful as attractants, but none of these substances was able to induce oviposition.

4. Tests with ammonium compounds and carbon dioxide have shown that, at the concentrations used, ammonia acted as an attractant. For oviposition to occur carbon dioxide had to be present.

5. Hydrogen sulphide was detected as a constituent of the fleece atmosphere of certain sheep. This substance, whilst having no attractive powers when used alone on sheep, increased the attractiveness of organic sulphur compounds.

6. Comparative tests with.ammonium carbonate-indole and ammonium carbonate-ethyl mer-captan mixtures have shown marked variations in the relative powers of these preparations to induce oviposition.

7. A distinction is drawn between stimuli which attract L. sericata to sheep and those which induce oviposition. Ammonia and various organic sulphur compounds enhanced attraction, but both ammonia and carbon dioxide were necessary for oviposition. It is suggested that some of these materials may be produced under natural conditions from the break-down of fleece keratin or by the bacterial decomposition of sweat and similar products in the fleece.

1. Seven organic sulphur compounds likely to be produced in the breakdown of cystine have been tested under field conditions as blowfly attractants. Two of them, ethyl mercaptan and dimethyl di-sulphide, when mixed with hydrogen sulphide or carbon dioxide formed powerful attractants for females of the blowflies Lucilia caesar (and L. illustris) and L. sericata.

2. In the majority of trials 10 ml. 0·2% ethyl mercaptan mixed with 10 ml. freshly saturated hydrogen sulphide solution was used as the control attractant. As a standard in field-trapping studies this material overcomes many of the disadvantages of meat baits.

3. Sodium carbonate, sodium bicarbonate, ammonium hydroxide, ammonium carbonate and indole, at various concentrations, did not activate the organic compounds. Furthermore, when added to the ethyl mercaptan-hydrogen sulphide preparation they did not increase its activity.

4. In the present experiments oviposition was rarely induced. When it did occur it was associated with the presence of indole.

5. L. caesar responded to attractants placed 2 ft. from the ground as well as to the same materials placed on the ground. Height alone, therefore, was not responsible for the fact that L. caesar did not respond to the attractants when they were placed on sheep.

6. The responses of blowflies to chemical attractants was linked with climatic conditions. Thus, under certain conditions Calliphora spp., particularly C. vomitoria, responded to these attractants.

7. The results obtained in the present investigation emphasize the attractive nature of sulphur-containing compounds and the possible importance of hydrogen sulphide and carbon dioxide as sensitizing agents of such compounds on sheep.