This study of the genetics of the Siamese fighting fish (Betta splendens) determined the following: (1) Sex ratios in Betta Splendens vary from near 100% females . . . to 100% males. (2) No genetic mechanism has as yet been detected by study of sex ratio in successive generations. It is concluded that sex determination is largely environmental although an attempt at direct control (e.g. by degree of crowding has given only a slight suggestion of success. (3) Large percentages of fish (100 out of 145) in certain spawns had defective, or no ventral fins. (4) Dorsal ray counts have varied from 8 to 18 although the description for the species gives 9 to 10. (5) Defect of ventral fins is strongly associated with increase in number of dorsal rays. (6) No indication is given of a correlation between the mean dorsal ray count of parents (or the spawn from which they came) and that of the offspring. There is a strong suggestion of a relation to conditions which affect early growth rate. (7) Breeding and statistical tests confirm the hypothesis that a single locus with two alleles (C, c) is responsible for the two major classes of color varieties, dark and cambodia. (8) A precise description of the range of phenotype (cambodia) produced by cc is given. (9) Breeding and statistical tests confirm the hypothesis that a sing locus with two alleles (V, v) is responsible for the three colors, steel blue (VV), blue (Vv) and green (vv) produced by iridocytes. (10) Breeding and statistical tests confirm the hypothesis that a single locus with two alleles (Ri, ri) controls the number of iridocytes on the body and fins, the homozygous recessive having relatively few. (11) A new pair of alleles (B, b) distinguish between the dark colored varieties (C_B_) and the ones with little dark pigment (but often bright red) (C_bb). Breeding tests show that b is not allelic to c but there has been no clear indication of a differential effect of B, b in cambodia (cc). (12) The gene b has given no indication of linkage, although the numbers are small, to C, V, or ri. (13) The gene (or genes) which eliminates red on the bodies of male cambodias is not recessive. (14) The gene (or genes) which eliminates red on the fins has variable penetrance and expressivity. (15) Tyrosinase has been demonstrated in the betta. This is presumably the first demonstration of the enzyme in fish. (16) Tyrosinase is present in cambodia, bright red and dark but not in colorless fins. (17) The data indicate that gene c does not reduce the amount of tyrosinase as compared with C but that differences in the oxygen consumption curves are due to an inhibitor of a stage subsequent to the oxidation of tyrosine. (18) Some inhibitor, although not necessarily the same as that in cc, is found in C_B_ fish. (19) Phenylthiourea strongly inhibits the action of fin tyrosinase and apparently protects the enzyme during the inhibition phase. (20) Similarities in the mode of inheritance of various characters in the betta and other fish are discussed. (21) The suggestion that red pigment in certain fish is hallochrome, finds some supporting evidence, although this may be purely circumstantial, from this work. (22) That the same pigment is controlled by different genes in different sites is shown several times throughout this paper.