Arsenic in ground water has been found above the maximum permissible limit in six districts of West Bengal covering an area of 34 000 km² with a population of 30 million. At present 37 blocks of these six districts by the side of the River Ganga are affected and about 800 000 people from 312 villages/wards are drinking arsenic contaminated water and amongst them at least 175 000 people are showing arsenical skin lesions. The source of arsenic is geological. We have analysed thousands of tube-well water samples from these six districts for four arsenic species namely, arsenite, arsenate, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA). We could detect no MMAA or DMAA in any of these samples. In urine, DMAA and MMAA are the predominant species along with arsenite and arsenate. The techniques we used for the determination and speciation of arsenic are: (i) separation of arsenite and arsenate from water by sodium diethyldithiocarbamate in chloroform followed by FI-HGAAS; (ii) spectrophotometry using Ag–DDTC in chloroform with hexamethylenetetramine as absorbing solution; (iii) ion-exchange separation of arsenite and arsenate from water followed by FI-HGAAS; and (iv) for analysis of inorganic arsenic and its metabolites in urine, FI-HGAAS was used after separation of the species by a combined cation–anion-exchange column. Total arsenic in urine was determined by FI-HGAAS after acid decomposition. The most toxic species, arsenite, is present in ground water at about 50% of the total arsenic level, and more than 90% of the total arsenic in urine is inorganic arsenic and its metabolites.