Serotonin is a biogenic amine that can function as a neurotransmitter, neuromodulator or neurohormone in invertebrates. Serotonin has been shown to play an important role in feeding and other behaviors in insects. An immunohistological technique was used to detect serotonin-like immunoreactivity (SLI) in the synganglia (brain) of the southern cattle tick Boophilus microplus, the lone star tick Amblyomma americanum, and the winter tick Dermacentor albipictus. SLI neurosecretory cells were identified in the protocerebrum region of the synganglia in all three tick species. These SLI neurosecretory cells project processes into the neuropils of the tick brain innervating the pedal ganglia, opisthosomal ganglion, olfactory lobes, and the multiple areas within the protocerebrum. SLI was absent in most of the nerves exiting the synganglia indicating that serotonin may not function as the neurotransmitter at the neuromuscular junction. The labeling pattern and number of SLI neurosecretory cell bodies is different between species. The differences are most prominent when comparing the three host tick A. americanum to the one host ticks B. microplus and D. albipictus.