High frequency deep brain stimulation of the fornix is currently being trialled as a treatment for the memory impairments associated with Alzheimer’s disease (AD), with mixed results. The fornix is a complex white matter bundle linking the hippocampal formation, basal forebrain and diencephalon. Clinicians have suggested that rather than delivering high frequency non-specific stimulation, broader efficacy might be achieved by selectively targeting the distinct neural activities present in the hippocampus, such as theta oscillations. Hippocampal theta oscillations have been strongly implicated in learning and memory processes and changes in theta power are often reported in patients with AD . In the present study, we used the Oscilltrack phase tracker to investigate if we can accurately deliver theta-locked fornix stimulation to healthy rats with implanted micro-drives while they ran on a linear track, and whether hippocampal theta can be selectively modulated by targeting distinct phases of the theta cycle. A high degree of phase tracking accuracy was achieved for the four theta phases targeted and we were able to phase-lock fornix stimulation to hippocampal theta at each phase. Furthermore, fornix stimulation produced a phase dependent change in the power and frequency of hippocampal theta oscillations in a manner that could not be explained by running speed. Our findings suggest that phase-locked fornix stimulation can be precisely delivered and provide a method to selectively modulate hippocampal theta activity. Further work will examine if these physiological changes translate into improved memory performance.