The formula of a new medium for mounting insects on slides is presented, having as base colophonium and copal dissolved in a mixture of alcohol, camphor, essence of turpentine and eucalyptol. The modification of this solvent by the addition of glacial acetic acid for purposes of preliminary fixation is also described. The new medium has the same permanence as those which it is designed to replace, and its cost is much less. Its characteristics and advantages follow. It gives an acid reaction. Being non-aqueous, it does not foster the growth of molds. It dries sufficiently slowly to permit dissection to be made. It clears specimens not previously treated with potassium hydroxide within 24 to 48 hours, thus making early study possible. The medium does not retract. Its refraction index at 26ºC is 1.467. Mounted material can be dismounted without heating. And finally, it offers no interference to staining when the stain employed are of acid reaction, and can be used in histological preparations.