Alien Appearance Ratios

December (1998) - January
Mission/Race	Sectoid	Snakeman	Ethereal	Muton	Floater
Alien Research	70%	10%	-	-	20%
Alien Harvest	50%	20%	-	-	30%
Alien Abduction	60%	-	-	-	40%
Alien Infiltration	60%	20%	-	-	20%
Alien Base	30%	20%	-	-	50%
Alien Terror	30%	-	-	-	70%
Alien Retaliation	60%	20%	-	-	20%

February - March
Mission/Race	Sectoid	Snakeman	Ethereal	Muton	Floater
Alien Research	60%	10%	-	-	30%
Alien Harvest	50%	-	-	-	50%
Alien Abduction	40%	-	-	-	60%
Alien Infiltration	50%	20%	-	-	30%
Alien Base	40%	20%	-	-	40%
Alien Terror	60%	-	-	-	40%
Alien Retaliation	30%	30%	-	-	40%

April - May
Mission/Race	Sectoid	Snakeman	Ethereal	Muton	Floater
Alien Research	30%	10%	-	30%	30%
Alien Harvest	30%	-	-	30%	40%
Alien Abduction	50%	-	-	-	50%
Alien Infiltration	20%	20%	-	50%	10%
Alien Base	10%	40%	-	30%	20%
Alien Terror	20%	60%	-	-	20%
Alien Retaliation	20%	20%	-	40%	20%

June - July
Mission/Race	Sectoid	Snakeman	Ethereal	Muton	Floater
Alien Research	20%	10%	-	40%	30%
Alien Harvest	10%	-	-	40%	50%
Alien Abduction	30%	-	-	-	70%
Alien Infiltration	10%	10%	40%	30%	10%
Alien Base	10%	40%	10%	20%	20%
Alien Terror	10%	20%	40%	20%	10%
Alien Retaliation	20%	20%	20%	20%	20%

August and Beyond
Mission/Race	Sectoid	Snakeman	Ethereal	Muton	Floater
Alien Research	30%	10%	-	30%	30%
Alien Harvest	30%	-	-	30%	40%
Alien Abduction	50%	-	-	-	50%
Alien Infiltration	20%	20%	10%	40%	10%
Alien Base	10%	20%	40%	20%	10%
Alien Terror	20%	30%	20%	20%	10%
Alien Retaliation	10%	10%	40%	20%	20%

Notes

• The first appearance rate block includes December 1998; however, the game starts in January 1999. It is included because the tables cover 2 month blocks.
• All rows except Alien Terror are unused in the January table; the non-Terror mission for January 1999 is hardcoded to be a Sectoid Alien Research mission in the same zone as X-Com's first base.
• All Retaliation rows are unused. Retaliation missions triggered by shooting down a UFO are performed by the race that crewed the UFO shot down. Scheduled Retaliation missions at the start of the month are evenly distributed among the races (20% each), regardless of month.
• All months beyond August 1999 use the August table.
• All values listed above are interpreted from probability tables in GEOSCAPE.EXE.
• Alien bases will appear upon completion of Alien Base and Alien Infiltration missions. Interfering with the mission will delay the base's appearance, but cannot prevent it. They will be crewed by the same race that performed the Alien Base or Alien Infiltration mission.
• Alien Supply missions will always be crewed by the same race as the base itself.
• The Cydonia mission has a pre-defined crew setup.

See also

• Alien Appearance Ratios (TFTD) for the appearance ratios from Terror From the Deep.