Fiber flax belongs to self-pollinating plant, but in the flowering period flowers are usually visited by many kind of insects such as honey bees, Bombus spp., Megachile spp., hover flies, butterflies, etc. The investigation was carried out from 1954 to 1957 to know how many extent viable foreign pollen will be brought and how many natural crossing will occur. Blue- and white- flowering varieties were planted in concrete frame located in the field of the Hokkaido National Agricultural Experiment Station. The flax flowers expected to open at next morning were castrated, and then a group was uncaged to expose insect visit, the second group was artificially pollinated next morning and the third was caged soon after the castration with ordinary fly screen. Natural crossing were estimated by number of blue flowered F1 plants in white flower variety planted alternately with dominant blue flower variety in 3 cm×3 cm. The results are summarized as follows : 1) In uncaged plot, 34.3 % and 31.7 % of castrated flowers grew capsule in blue- and white- flower variety respectively. If there were no injury caused by castration, more capsule will grow. By following formula the possibility of capsule setting was estimated as 45.2 % and 41.0 % respectively. Possibility of capsule setting=(No. of capsules in uncaged plot)/(No. of castrated flowers in uncaged plot×(No. of capsules in artificially pollinated plot)/(No. of castrated flowers in artificially pollinated plot)) 2) In caged plot, 9.0 % and 3.8 % of castrated flowers grew capsule. The capsule setting in caged plot was very fewer than in uncaged plot. These results show pollens which fertilize the castrated flower are mainly brought by insects. 3) Daily capsule setting of castrated flower was very variable, 3.8 %-77.5 %. The flowers which opened under low temperature about 18°C, grew a few capsule by few insect visit. 4) Natural crossing were 27.94 %, 5.38 % and 1.62 % at original mother plants, capsules and progeny seeds basis. Crossed capsules had 1.3 croseed seeds of 4.4 grown seeds in average. In conclusion, to flax flower many foreign pollens are supplied, by insects, but natural crossing are relatively low because the flower is pollinated preferentially by her own pollen.