AIB is a set of two interneurons with cell bodies situated in the lateral ganglia; it is one of the main classes of integrating neuron for the receptors of the amphid sensilla. Processes enter the ventral cord from the cell bodies via the amphid commissures and project anteriorly into the nerve ring. They then run round the nerve ring close to the posterior face, in close association with the processes of AIA. When they reach the dorsal midline, at the point where the processes of AIA terminate, they turn and run anteriorly for about 2.5 um at right angles to the orientation of the neighbouring processes in this region (b). They then turn again and continue running round the ring, but now in close association with the proximal processes of RIM near the anterior surface of the ring. They eventually reenter the ventral cord and finally end in the region of the ventral ganglion. AIB is presynaptic only on these distal regions of its processes; the predominant postsynaptic partners are RIM (a, c, d), AVB (c), RIB (a, g) and SAAD (d). Because of the unusual shift of position that occurs on the dorsal mid-line, the distal and proximal regions of AIB reside in different regions of the ring neuropile and the synaptic inputs are, thererore, different for these two regions. The proximal regions receive essentially the same synaptic input as AIA (except for synapses from AIA), with AIB being the second postsynaptic element in dyadic synapses. Most of the synapses appear to be symmetrical, although some have a bias towards AIA (e). The main synaptic inputs in these proximal regions are from AIA (*a, *b), ASE, ADL (*c), ASH, AWC (*b), ASG (*b), AIZ (*g), ASK (*d) and ASI. The main synaptic inputs on the distal branch are from AIZ (*e, *f), DVB (*a), DVC (*c), RIM (f), RIB and FLP. AIB has gap junctions to DVB, DVC, RIG (h), AFD, RIS and RIV (*h). Magnifications: (a, h) x 25500, (b-g) x 12750.