Look! Up, up! Es ist ein Ultralight!

Lufthansa pilots fly around in them on busman's weekend holidays.

Pfennig-pinching workers find their relatively low cost well within their budgets.

Invented in the United States, these personalized, motorized sets of wings with open pendulum seats - ultralights - are the latest craze to sweep West Germany.

This year ultralights came sufficiently into their own to have entries in the Hannover Air Show. And they will get an air show all their own in Bad Pyrmont Oct. 14-17. Some 69 local ultralight clubs already exist here, and the fad is spreading rapidly among the German Aero Club's 70,000 members.

The sport is growing so fast, says Lothar Alexy of the flight safety section of the German Aero Club, that the club is ''overwhelmed by the wave'' of interest. Any pilot who wants to convert his general three-dimensional aerodynamic steering knowledge into basic familiarity with the new puddle-jumper can take a two-hour course the club now offers. What takes the most getting used to is the much slower response time (in comparison with larger craft) between shoving the stick and getting the plane's reaction to it.

In terms of safety, the German penchant for order is paying off. There has been only one fatal ultralight accident compared to last year's 40 or 50 fatalities in the United States and this year's eight in Britain.

The West German solution is a mix of regulation, voluntary compliance with certain standards - and sheer common sense.

Until May 15 of this year ultralight flying was illegal (though a number of red barons still did it on the sly, insofar as such a conspicuous sport can be done secretly). Since May 15 it has fallen under Transport Ministry regulations governing ''manned unregistered flying'' in hang gliders as well as in the dragonfly-like aluminum-and-dacron ultralights. This fall the new sport will also fall under special guidelines drafted by the German Aero Club and the German Hang Gliding Association and approved by the Transport Ministry.

Under these (still tentative) guidelines the new-old Wright brothers-type seat-of-the-pants aircraft must have a minimum wing surface of 10 square meters, a maximum weight of 100 kilos (220 lbs.), a maximum gas tank of 20 liters, a minimum lift-off speed at full weight of 1 meter per second, and full steering capacity at a minimum speed of 45 kilometers per hour. Only solo planes are allowed.

So far, says Mr. Alexy, an estimated 2,000 aircraft more or less fitting this description exist in West Germany (as compared with 15,000 in the US).

Despite the cost of fuel - 2.10 marks (88 cents) per liter - the purchase costs (10,000 to 20,000 marks, or $4,000 to $8,000) and upkeep (storage in a spare garage corner and transport on top of a car) are easily affordable.