Low-temp. crystal structure studies on small cyclopropyl-conjugated unsatd. hydrocarbons reveal significant differences in bond lengths between C atoms mainly within the 3-membered rings but also for adjacent C-C single bonds. The strongest influence of conjugation was obsd. for spiro[2.4]hepta-4,6-diene, where the difference in the lengths of 3-membered ring bonds is 0.050 .ANG. (x-ray). The same effect is also obvious from ab initio calcns. at HF/6-31G(d) (to a minor extent) and MP2/6-31G(d) levels, as shown by a difference anal. of bond lengths which was performed to cancel out systematic errors and differences for both methods. Static difference electron d. maps were derived from the exptl. data by multipole refinements. These maps showed shifts of electron d. to exocyclic locations within the planes of the 3-membered rings and significant bond ellipticities for the adjacent single bonds. For 1,1-diethenylcyclopropane a nonsym. conformation was found in the crystal lattice, very close to the global min. structure calcd. by ab initio methods.