Transcatheter interventions for structural heart disease demand real-time visualization of catheter devices and their relationship to cardiac anatomy. Co-registration of x-ray fluoroscopy with echocardiography has been proposed to provide the necessary device and soft tissue visualization for these procedures. Development of real-time 3D x-ray/echo registration systems with device tracking has been hampered by the lack of a suitable test model. This study presents a phantom that is compatible with x-ray, CT, transthoracic (TTE), and transesophageal echo (TEE) for testing the feasibility and accuracy of new registration solutions. The phantom consists of a 20.3-cm diameter, 15-cm tall cylindrical shell with acoustic windows for TTE and an access port for a TEE probe. The interior contains 24 dual-modality targets, 5-mm in diameter, suspended in a three-turn helix occupying a volume that is similar to an adult heart. An ultrasound-compatible, tissue-mimicking slurry medium fills the remainder of the phantom. The dual-modality targets are agar based with barium sulfate (BaSO4) powder and glass beads added to generate contrast in both x-ray and ultrasound. Appropriate concentrations of these additives were determined experimentally with contrast measurements in x-ray, CT, and ultrasound. Selected concentrations were 150 mg/mL BaSO4 and 100 mg/mL of 53-63 μm diameter glass beads. Average target contrast (± SD) was 16% ± 2% in x-ray fluoroscopy (90 kV) and 1805 ± 99 HU in CT (100 kV). In ultrasound, target CNR was 4.30 ± 0.62 in 2D B-mode and 4.03 ±1.06 in 4D-mode images acquired at a center frequency of 2.8 MHz.