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Yee & Inouye (1982) described a new electrophoretic procedure for sizing the prokaryotic genome. In principle, a digest of genomic DNA with a six-base recognition restriction enzyme is resolved by electrophoresis in agarose. The DNA in a strip of the agar is re-digested with a second such restriction enzyme and resolved by electrophoresis at right angles to the original current. Appropriate molecular weight markers are included at the second stage. Fragments are then sized with reference to the markers and summed to give the genome size. This procedure is complementary to, and consistent with, DNA reassociation curves, analyses of minimum DNA contents or sizing of electron micrographs (see Yee & Inouye, 1982).We have applied this technique to single strains of Desulfovibriogigas and D . vulgaris, in which we have also observed plasmids by conventional electrophoresis; we have also examined 12 other strains of Desulfovibrio for plasmids and observed them in two. METHODSDesulfovibrio gigas (NCIMB 9332), kindly provided by Dr E. C. Hatchikian, (CNRS, Marseille, France), was grown in a simple wier-type chemostat ( V , 540 ml) at D = 0.025 or 0.03 h-under 0,-free N2 at 28 "C in lactatesulphate medium C (Postgate, 1984) modified so that the population was limited by the N supply by including only 0.03 g NH4Cl 1-I. Periodic cultural and visual checks were made for contamination (Postgate, 1984); none was present in any of the experiments reported here. Desulfovibrio vulgaris Hildenborough (NCIMB 8303) was grown in batch cultures (20 or 200 ml) to stationary phase under N, and was taken as N-limited. Desulfouibrio gigas and some other desulfovibrios (Table 2) were grown in normal medium C with extra NaCl as necessary. Escherichia coli

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