In this paper we report the first experimental evidence for the existence of a liquid-liquid phase transition(LLPT) in the transition metals. The transition is evident from discontinuities in the melting slope measured at high pressure. The discovery of LLPTs constitutes the strongest evidence for the presence of polymorphic structures in transition metal liquids at high pressure, and confirms the thesis that they are responsible for the relatively low melting temperatures, low melting slopes, and unusual phase diagrams. PACS number(s): 62. 50, 64.70.Ja, 64.70.Dv In earlier reports [1,2] we proposed that, in order to explain the apparent discrepancy between the diamond anvil and shock wave melting measurements of Mo and Ta, a liquid-liquid phase transition(LLPT), or liquid-glass transition, had to be present in the phase diagram. LLPTs are characterized by liquids of the same composition, but with different densities, and different atomic configurations, often referred to as polymorphs.In this paper we present the first experimental evidence for LLPTs in transition metals.It is now well established that elements with partially filled p-electron valence shells have directional bonding and exhibit an LLPT. Some examples are carbon [3,4], silicon [5,6], nitrogen [7,8] and phosphorous [9,10]. Bellisent et al.[11] interpreted the structure of liquid As, obtained from neutron diffraction experiments, as the consequence of a strong Peierles distortion in the half-filled 3p-band [12].The partially filled d-electron bands of transition metals also have the potential for a Peierles distortion, and directional bonding. The low melting slopes of transition metals has been attributed to directional d-electron bonding leading to the formation of 2 preferred local structures with five-fold icosahedral or polytetrahedral short range order(ISRO) resulting from Peierls/Jahn-Teller(P/JT) distortions [12,13]. Evidence for the presence of ISROs in transition metal liquids is now well documented and are referenced in our earlier reports [1,2]. ISROs introduce geometric frustration inhibiting solidification, and their presence as soluble impurities increases the communal entropy, further favoring the liquid stability and lowering the freezing temperature. Since ISROs are denser than the pure atomic liquid, pressure enhances the concentration of local structures. Therefore, it should come as no surprise, that pure non-alloyed transition metal liquids, with partially filled d-bands, exhibit polymorphic behavior, and a pressure-induced LLPT.The scarcity of experimental evidence for LLPTs in transition metals is at least partially due to the relatively high temperatures characteristic of these liquids. With the advent of the laser-heated diamond-anvil-cell technique it has now become possible to access melting curves of transition metals up to nearly 200 GPa and 4000K. The experimental method employed on a wide variety of solids, including on the transition metals, has been described and referenced in earlier reports [14][15][16][17]....