There are, at least, 11 distinct single bands and one real doublet in the region 2B1-2--2C1-2 of the X-chromosome of Drosophila melanogaster. This figure coincides with that in Bridges' revised map with most of the "doublet" bands being artifacts. Three puffs appear in the region. The early ecdysone-specific puff 2B5-6, small at PS(puffing stage) 1, increases sharply at PS 2. The late ecdysone-specific puff 2C1-2 appears at PS 4--5. At PS 9--10, when 2B5-6 disappears completely and 2C1-2 decreases, a third puff at 2B11 appears. None of these puffs is active at PS 11. Morphological analysis of puff appearance and autoradiographic study of 3H-uridine incorporation into chromosomes carrying rearrangements within the 2B region suggest that the early ecdysone-specific puff derives from bands 2B5 and may be 2B6, while the neighbouring bands 2B1-4, and 2B7-10 do not show appreciable transcription at the investigated stages. There are 42 mutations affecting viability in the region where the 2B5-6 puff is located. The mutations belong to 6 complementation groups; two of the groups dor and swi, are independent while the rest are overlapped by several lethal mutations (overlapping complex.) Mutants of the different groups have series of similar characteristics: temperature sensitivity, dose sensitivity, larva-pupal lethality and similar morphological abnormalities. It can be assumed that there is a functionally linked cluster of genes within the region 2B. Complementation groups br, rbp, l(1)pp-1, l(1)pp-2 (overlapping complex) have been located by rearrangements in very narrow cytological limit 2B3-4--2B5 that is in the area of developing puff. Two other loci dor and swi are situated some to the right of 2B5.