Alfons Klein was transferred to the Legion Condor on 6 August 1936.

In Spain he served in the 4 Staffel of J/88.

On 26 September 1936, the German pilots accounted for the destruction of three enemy aircraft - a Vickers Vildebeest for Leutnant Wolf-Heinrich von Houwald and a Breguet XIX each for Oberleutnants Ekkehard Hefter and Klein.

On 2 March 1937, the Chief of Staff, Oberstleutnant Dr.-Ing. von Richthofen’s plans for reorganisation and rotation began to start when four of the original pilots, Oberleutnant Hannes Trautloft, Leutnant Wolf-Heinrich von Houwald, Unteroffizier Erwin Sawallisch and Oberleutnant Klein, were sent back to Germany.

In 1938, he served in Fliegergruppe 30.

Klein was appointed Staffelkapitän of 3./JG 433 on 1 November 1938.

He was killed on 18 February 1939, when the Ju 53/3m tri-motor transport he was co-piloting crashed due to severe icing in a snowstorm at Falkenau, 30 km north-east of Eger while flying from Manching to Berlin.
The aircraft was from I./JG 433 and killed together with Oberleutnant Klein was the pilot Unteroffizier Mandelkow and the nine passengers, Leutnant Möltjenhof, Feldwebel Löhlein, Unteroffizier Esser, Unteroffizier Ludwig, Unteroffizier Pohl, Unteroffizier Buck, Unteroffizier Erhardt, Obergefreiter Mandel and Gefreiter Romen.
Purportedly, they were going to attend the Berlin Motor Show.

At the time of his death, Klein was credited with 1 biplane victory, this one being claimed in the Spanish Civil War.

Claims:

Kill no.	Date	Time	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1936
1	26/09/36		1	Breuget XIX	Destroyed	Heinkel He 51B-1		Spain	4.J/88

Biplane victories: 1 destroyed.
TOTAL: 1 destroyed.

Sources:
Aces of the Legion Condor – Robert Forsyth, 2011 Osprey Publishing, Oxford, ISBN 978-1-84908-347-8
Aces of the Luftwaffe
Aircrew Remembered
Luftwaffe Officer Career Summaries – Henry L. deZeng IV and Douglas G. Stankey
The Legion Condor - Karl Ries and Hans Ring, 1992 Schiffer Publishing, ISBN 0-88740-339-5
The Luftwaffe, 1933-45
Additional information kindly provided by Leo Etgen and Alfredo Logoluso.