Therapeutic ultrasound is a deep heating modality often used to produce vigorous heating (≥4°C Δ) in tissues. The vigorous heating effects of 3 MHz therapeutic ultrasound have only been tested to a 2.5 cm depth, but its maximal depth of producing vigorous heating has yet to be established. Objective: To investigate the tissue temperature change produced by a 3 MHz ultrasound treatment at depths of 3 and 3.5 cm in the human triceps surae muscle group. Design: Randomized control design. Setting: Therapeutic modalities research laboratory. Patients or Other Participants: Twenty healthy college-aged participants (male = 13, female = 7; age = 23.4 ± 1.31; calf subcutaneous fat thickness= 0.6 cm ± 0.2 cm). Participants were randomized into treatment (n = 15) and sham (n = 5) groups. Participants were blinded to their group assignment. Interventions: Two MT-26/6 needle thermocouples were inserted into the left posterior triceps surae at depths of 3.0 ± 0.1cm and 3.5 ± 0.1cm from the skin's surface. Participants in the treatment group received a continuous 3 MHz ultrasound treatment at 1.4 W/cm2 for 8 minutes with 10mL of 100% ultrasound gel as a coupling medium. Participants in the sham group received the same treatment parameters, but the ultrasound device was not turned on. The Omnisound 3000 ultrasound device (ERA = 4.2cm2, BNR = 3.0:1) was used for all treatments. A 15 cm2 template was used to ensure a constant and proper treatment size. Baseline temperature (TB) was established by taking a mean of intramuscular tissue temperature (TIM)for five minutes before the treatment and TIM were recorded every 10 seconds throughout the experiment session. Participants marked a visual analog scale (VAS) indicating heat sensation at pre-treatment and post-treatment. Main Outcome Measures: A 2 x 2 x 2 (probe depth x condition x time) ANCOVA with TB used as a covariate analyzed the difference in TIM. We only used the time points of baseline and final TIM for our analysis as we are only interested in the change in TIM from beginning to end of the ultrasound treatment. Descriptive statistics for TIM and VAS for heat sensation were computed as post-treatment minus pre-treatment for each condition and probe depth. Results: There was a significant difference in TIM between the conditions at the different probe depths from the beginning and end of the ultrasound treatment (F1,15 = 7.35, p = 0.016). The mean changes in TIM for each condition at each probe depth were: sham 3cm = -0.4 ± 0.3°C, sham, 3.5cm = -0.2 ± 0.3°C, treatment, 3cm = 4.4 ± 0.2°C, treatment, 3.5cm = 3.5 ± 0.2°C. Mean VAS scores for each group were: sham = 0 ± 0mm and treatment = 71.8 ± 11.8mm. Conclusions: At 3cm deep into the posterior calf, the Omnisound 3000 using a 3 MHz treatment produced vigorous heating (≥4°C Δ). Moderate heating (2-3°C Δ) occurred at 3.5cm deep into the calf. Three MHz ultrasound may be used to heat tissues deeper than previously theorized, but it does, however, create a moderately high level of heat sensation for the patient.