The crystal structure of the potassium salt of penicillin V has been studied by the X-ray crystallographic methods. Crystal data are as follows; potassium 3,3-dimethyl-7-oxo-6-phenoxyacetoamido-4-thia-1- azabicyclo[3.2.0]-heptane-2${alpha}$-carboxylate, $K^+{cdot}C_{16}H_{18}N_2O_5S^-$, $M_r$= 388.5, triclinic, Pl, a= 9.371 (1), b= 12.497 (2), c= 15.313 (2) ${AA},;{alpha}= 93.74;(2),;{eta}=99.32;(1),;{gamma}=90.17;(1)^{circ},;V=1765.7;(2);{AA}^3$, Z=4, $D_m=1.461;gcm^{-1},;{lambda}(Cu;K{alpha})=1.5418;{AA},;{mu}=40.1;cm^{-1}$, F(000)=808, T=296 K. The structure was solved by the heavy atom and difference Fourier methods with intensity data measured on an automated four-circle diffractometer. The structure was refined by the full-matrix least-squares method to a final R= 0.081 for 3563 observed $[I_0{geq}2{sigam}(I_0)]$ reflections. The four independent molecules assume different overall conformations with systematically different orientations of the phenyl groups although the penam moieties have the same closed conformations. There are intramolecular hydrogen bonds between the exocyclic amide nitrogen and phenoxy oxygen atoms. The penam moiety is conformationally very restricted although the carboxyl and exocyclic amide groups apparently have certain rotational degrees of freedom but the phenyl group is flexible about the ether bond despite the presence of the intramolecular N-H${cdots}$O hydrogen bond. There are complicated pseudo symmetric relationships in the crystal lattice. The penam moieties are related by pseudo 20.5 screw axes and the phenyl groups by pseudo centers of symmetry. The potassium ions, related by both pseudo symmetries, form an infinite zigzag planar chain parallel to the b axis. Each potassium ion is coordinated to seven oxygen atoms in a severely distorted pentagonal bipyramid configuration, forming the infinite hydrophilic channels which in turn form the molecular stacks. Between these stacks, there are only lipophilic interactions involving the phenyl groups.