We describe experiments on plastic flow of solid 4 He near the first order bcc-hcp transition. We find that the resistance to shear of the solid diminishes strongly near the transition. We are able to distinguish between plastic flow due to vacancies and due to dislocations, and show that the critical behavior of the solid near the transition is controlled by vacancy diffusion. The enhancement of the diffusion is consistent with a coupling of the vacancies to a phonon which softens near the transition. Possible implications of our findings on the understanding of melting are discussed.[S0031-9007(98)06732-5] PACS numbers: 67.80.Mg, 62.20.Hg, 64.70.Dv In an experiment aimed at measuring the plastic flow properties of solid He, we found that the shear resistance of the solid decreases dramatically as the temperature nears that of a bcc-hcp phase transition [1]. Over a temperature interval of 0.1 K, the plastic flow rate increased by 1 1 2 orders of magnitude. In that experiment, the flow of solid took place by diffusion, yielding a value of the diffusion coefficient at the transition typical of a liquid. The loss of resistance to shear is the most prominent feature associated with melting. Theories of bulk melting proposed over the years rely on the sudden proliferation of crystalline defects in order to destabilize the lattice [2] so that the shear resistance vanishes. These include point defects (vacancies or interstitials) or line defects, namely, dislocations. However, this scenario was never confirmed experimentally, as the equilibrium density of defects in conventional solids is rather small, and it is impossible to approach the transition close enough to see critical behavior of the defects. The first order bcc-hcp structural phase transition in solid 4 He offers several advantages in this type of study. First, it is weakly first order (the entropy change per particle is 12 times less than that associated with melting [3]), and consequently the temperature interval where pretransition effects can be observed is wider. One can therefore resolve how the shear resistance changes as the phase transition is approached. Second, the estimated density of vacancies [4] in 4 He is significantly higher than what can be achieved in conventional solids even at the melting temperature. This condition of having a high defect density is another indispensable feature of the models of melting. Third, macroscopic single crystals of very high quality and purity can be grown. Furthermore, by investigating the plastic flow rate over a wide range of applied stresses, we can decide which type of defect, vacancies or dislocations, is responsible for the critical behavior near the transition (at low stress plastic flow results from vacancy diffusion, while at higher stress moving dislocations determine the rate). Thus, solid He is an excellent model system in which effects typical of the approach to the melting transition, such as loss of shear resistance and high defect density, can be conveniently studied. In this Letter we repo...