The characteristics of the possible indices of the yellowfever mosquito, Aedes aegypti (L.), are examined. While all 4 indices give similar population values, the “premises index” (the percent of the inspected premises that are infested), is considered the best because it has less variation and varies linearly with the infestation. The “block index” (the percent of the inspected blocks that are infested), which is useful in setting up spray operations, has little variation but does not vary linearly with the infestation. Careful sampling techniques are required to give valid indices. Censuses, systematic samples. or random between the premises index and the block index and by the fact that the 2 indices are highly correlated, 1 can be transformed to the other. This means that when only the population level is required, sample sizes can be minimal. When populations are low and eradication of foci is being carried out, examination of every premise is required. The relationship of premises indices and block indices can be used also to locate areas with unusual A. aegypti populations.