Four organoiodone fungicides[4-chlorophenyl-3-iodopropargyl(IF-1000), 3-iodo-2--propynyl butyl carbonate(IPBC), 3-bromo-2. 3-diiodo-2-propenylethyl carbonate (EBIP), and 2, 3, 3-triiodoallyl alcohol (TIAA)] were proved to possibly be used as an alternative to tri-n-butyltin oxide (TnBTO) and conventioal wood preservatives, based on the Japanese standardized methods for their fungicidal and termiticidal efficacy. Fungal decomposition and physico-chemical degradation of these fungicides were also examined in the laboratory. The important results obtained are summarized as follows. In PART I , fundamental fungicidal and termiticidal efficacy and their synergistic effects with organophosphate termiticide(chlorpyrifos), synergist and stabilizer were described. The order of their fungicidal effect in single use was TIAA(0.5%) $gt; IPBC(0.75%) $gt;IF-1000(1.0%)≥EBIP($gt;1.0%) . Fungicidal efficacy of TnBTO was equal to 1F-1000. Mixing of all fungicides with chlorpyrifos and surface active agent gave a synergistic fungicidal effect against Coriolus versicolor. However, the effect was not always found in other test fungi. Only IPBC gave a desirable synergistic effect against. all fungi tested by mixing with surface active agent, IF -1000 and TnBTO caused an undesirable antagonistic effect against Tyromycer palustris when mixed with chlorpyrifos. EBIP showed a synergistic and/or additive effect against T. palustris and Serpula lacrymans, only when they were mixed with commercial synergistic and stabilizing agents. In PART II, fungal decomposition of organoiodine fungicides in shake culture was discussed . Lowering of pH during incubation and the decomposition rate of fungicides were closely related to each other. Most of organoiodine chemicals were degraded at various extents by fungal metabolic activities. The degrading ability of C. versicolor was found in intracellular and extracellular components. However, those of T. palustris gave a poor degradation of the fungicides tested. Detoxifying activity of this fungus occurred exclusively in-cell wall components. S. lacrymans was positioned between the two fungi, in its degrading activity of test fungicides. In PART III, the physicochemical degradation of organoiodine ungicides was described. Chemical bond energies of carbon-iodine and carbon-bromine in these compounds were comparatively lower than those of the other chemical bonds. Thus, these chemicals could be decomposed by exogenous energies, such as ultraviolet energy, causing the decline of fungicidal efficacy. The iodopropargyl-containing IF-1000 and IPBC were converted to propargyl compounds by iodine releasing through carbon-iodone bond cleavage. TIAA, in the similar manner, was convened to 1-propanol. However, these chemicals were not degraded by thermochemical exogenous energy. Although some of the four organoiodone fungicides tested have some defects such as degradation by fungi and photoirradiation, they showed the over-all good fungicidal efficacies. Their relatively lower resistance to C. versicolor might be overcome by mixing with chlorpyrifos which is specifically toxic to this fungus. Photodegradation of the chemicals might not be considered as fault in their actual application to timber treatments, because the degradation has been proved to be limited on the surface of the treated woods.