Calcium ions play important roles in the adaptation of auditory hair cells, and there is evidence that they are involved in modifying the sensitivity and adaptation of a variety of vertebrate and invertebrate mechanoreceptors. However, there is little direct evidence concerning the concentration changes, signaling pathways or ultimate effects of these proposed modulatory mechanisms. We measured receptor potential, receptor current and action potentials intracellularly during mechanotransduction in a group of sensory neurons of the spider Cupiennius salei, which possesses low-voltage-activated calcium channels. Simultaneously, we elevated intracellular [Ca2+] by UV light release from cage molecules, and observed increases in [Ca2+] as changes in calcium-sensitive dye fluorescence. Increases of 10–15% in [Ca2+] caused reductions of approximately 40% in receptor potential and approximately 20% in receptor current. Mechanically evoked action potential firing caused much larger increases in [Ca2+], and the firing rate fell as [Ca2+] rose during mechanical stimulation. Release of caged calcium just before mechanical stimulation significantly reduced peak firing. Dose–response measurements suggested that the binding of one or two intracellular calcium ions per channel reduces the probability of the mechanotransduction channel being open. Our data indicate that calcium regulates sensitivity in these mechanoreceptor neurons by negative feedback from action potentials onto transduction channels.