Few ecological studies of the solitary bees of the genus Andrena Fabricius have been made in Britain, which is rather surprising as most of the sixty or more species are comparatively large insects, many are abundant on flowers, and the individual species are not difficult to separate. On the other hand, our general knowledge of the genus and distribution of the species is quite substantial, as, in addition to the outstanding paper by Perkins (I9I9), there are a large number of records in the county lists and in notes scattered throughout the literature. From these sources it is known that some species are found only in certain types of localities and ecological formations, others have a limited geographical distribution, and that a considerable number are rare or erratic in appearance. In addition, observations have been made upon the flowers visited by the different species: here again, hymenopterists have emphasized the special peculiarities and limited choice of flowers visited even by some of the commoner species. The information available, although extensive, is limited in depth and mainly descriptive without a quantitative basis. With the object of attempting to gain more precise knowledge this work was begun during I94I, the present paper being of an introductory nature. Many factors govern the distribution and abundance of Andrena species. These may include edaphic factors such as geological origin, texture and particle size of soil, its properties of water retention, all of which will have their influence in any particular soil mass upon its drainage, cohesion and physical stability, of importance to fossorial Hymenoptera in the selection of burrow sites. The sources of larval food-pollen and nectar-are the next concern of the bee after completion of its burrow, and it is with the first of these that I shall be concerned. Here again, to my knowledge, we do not know what are the relative proportions of these two constituents required in the larval diet, but this is a physiological rather than an ecological question, and no attempt will be made at present to supply an answer. Even if only these two categories of soil and larval food are considered, it is perhaps not too sweeping to state that human operations, e.g. drainage, deafforestation and the expansion of agriculture, have had a greater effect upon the distribution and population of fossorial Hymenoptera than upon other orders of insects. The present Andrena fauna can therefore be considered to be in a sense 'artificial' and in a state of instability as regards the numerical representation of species and individuals in each species. A study of the plants foraged for may therefore give some guide as to the adaptability of a species and, by correlation with changes in the flora, may enable an interpretation of the present distribution of the species to be made. Previous workers who have recorded the flowers visited by Andrena species have not always specified the sex of the individuals. It is highly probable that the males will visit any flower as long as it is a source of nectar, so that not much significance can be attached to observations made upon this sex, although in some species the males tend to frequent the same flowers as the females. Again, simple observations of flowers visited by the females are of less value than if the evident object of the visit, i.e. whether for pollen or nectar, had also been recorded. Furthermore, it is much more important to know not what flowers the females of a species will visit, however long the list, but what are their normal preferences at any one time or in any particular locality. Accurate counts of relative frequencies of visits made to different plant species furnish valuable results, but this method, which has been used by many early workers in classical studies upon the honey bee (see Clements & Long (1923) for a review of the early work), is laborious and difficult to apply to Andrena, as the females with their more rapid flight are difficult to follow, the species are much less numerous in individuals than the honey bee, and it would be wellnigh impossible to make counts on all the plants likely to be visited in a locality. Admittedly, as we have no method at present for the determination of the source of a sample of nectar, individual counts of visits are necessary to ascertain nectar preferences; and as I have already shown (I945) that visits may be made to flowers for pollen alone, it is quite probable that nectar may be taken from flowers either when the pollen is not available or it may be ignored. On the other hand, the wide use of the technique of pollen analysis during the past few decades has