Recent evidence indicates that myomorph rodent species use ultrasonic calls as communication signals. The range over which sound communication signals may travel and the ease with which they may be localized depends on their intensity and structure and the structure is made. It is concluded that rodent calls are mainly within the range 20–100 kHz and not longer than 300 msec, except for some rat calls which last up to 3 sec. Intensities may be as high as 103 dB SPL (at 10 cm) in pups and 86 dB SPL (at 5–30 cm) in adults. Bandwidths between 1–104 kHz are found. High frequency sounds are attenuated with distance more than lower frequency sounds, mainly by atmospheric attenuation, ground attenuation and scattering. These effects are not all linear so it is difficult to predict how far rodent sounds may travel without making measurements under the conditions in which sounds are known to be produced by rodents in the wild. It is shown that there is little attenuation due to scattering from vegetation in a wood inhabited by woodmice. But in grass or wheat where field voles may live, sounds above 20 kHz are rapidly attenuated. Attenuation may be much less in rodent runs and burrows and this is being studied by a new spark technique.